Glossary

There are many technical terms you might come across as you learn about repair, how things work, and electrical and electronic fundamentals, but don't let that put you off. Here you will find simple definitions of most of them.

Summary

A primary aim of the Restart Wiki is to assume no previous knowledge, only a willingness to learn, and so we try to introduce and explain technical terms as we go along. But you might dip in and out or read the end of an article before the beginning, or you might consult other sources containing unfamiliar terms.

So this page gathers together as many technical terms as possible that you might come across in your fixing-related research, cross-referenced and linked in to main wiki articles where relevant.

Glossary of Terms

0-9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Term	Meaning
0 - 9
0-Day	A zero-day is a vulnerability that is found and already being exploited by malicious actors before any fix is available, giving potential victims no time at all (zero days) in which to protect themselves. Also known as an 0-Day (pronounced "oh-day").
2FA	Two Factor Authentication (2FA) and Multi Factor Authentication (MFA) are authentication schemes requiring a user to pass two or more tests, different in kind, in order to prove their identity. These are usually chosen from something you know (for example a password), something you have (a physical token) and something you are (a biometric such as a fingerprint or facial scan). Such a sytem is much harder to compromise than a simple password login.
3 Phase	Three phase is a system of power transmission in which three wires carry AC phased like three blades of a propeller. Consequently they balance out and there is no need for any wires to carry the return current. As a result, three wires can carry three times as much power as two wires in a conventional single phase circuit.
A
AC	AC or Alternating Current is an electrical supply which reverses direction many times per second. See AC and DC in the Wiki.
AC/DC motor	An AC/DC or Universal motor is a type of electric motor that runs equally well on AC or DC. See DC and Universal (AC/DC) Motors in the Wiki.
Adware	Adware is a type of malware which presents you with unwanted advertising that you never knowingly agreed to receive.
AF	AF or Audio Frequency is a frequency within the audible range of approximately 20Hz - 20kHz.
Aftermarket	Aftermarket refers to spare parts, consumables or accessories for an item not manufactured and sold by the item's manufacturer, but by a third party.
AHCI	AHCI (Advanced Host Controller Interface) is the logical access method used by an operating system to access SATA drives to exploit their capabilities to the full. It supersedes Legacy or IDE mode.
AM	AM or Amplitude Modulation is a method of modulation in which the amplitude of an RF signal is modulated by the AF signal to be transmitted. See Amplitude Modulation in the Wiki.
AMD64	AMD64 is a 64 bit CPU architecture created by AMD as an evolutionary development of the Intel x86 32 bit architecture.
Amp	The Amp is the unit of electric current, i.e. the quantity of electricity passing, a bit like the amount of water flowing in a river. Named after French mathematician and physicist André-Marie Ampère, remembered for his early work on electromagnetism. See Electric circuits, volts amps watts and ohms in the Wiki.
Analogue	Analogue is a method of representing a signal by a voltage (or a current) which is directly proportional to the value of the signal at any instant. For example, the output of a microphone is a voltage which continuously varies in direct proportion to the pressure of the incident sound wave.
Anode	An Anode is a positive electrode.
ASIC	An Application-Specific Integrated Circuit or ASIC is an integrated circuit designed with a very specific application in mind, unlike most which are designed to be as generally applicable as possible in order to leverage economies of scale. Often the design will be built largely out of a library of standard logic blocks, such as a CPU core, RAM and various peripheral interfacing blocks.
ATA	ATA or AT Attachment is an interface standard primarily used for the connection of storage devices and which has gone through a number of iterations, starting with the original IBM PC/AT. Earlier versions used a parallel interface and are now known as PATA but this has now been superseded by a serial version known as SATA.
B
B7G	B7G was a 7-pin base used on miniature all-glass valves.
B9A	B9A was a 9-pin base used on miniature all-glass valves.
Ballast	Whereas a conventional resistor only draws a current equal to the voltage divided by the resistance, in the case of a LED or a lamp utilising an electrical discharge through a gas or vapour (such as a fluorescent tube) the current increases rapidly once a certain voltage is reached. The current must therefore be limited by the circuit driving it. There are several ways this can be achieved but they are all referred to as a "ballast".
BC	BC or Bayonet Cap is a type of lamp base fitted with a push-and-twist action. There are several sizes but the 22mm diameter size is much the commonest. See Types of bulb in the Wiki.
Bimetallic Strip	A bimetallic strip consists of two strips of different metals fixed together. When heated up, one strip expands more than the other, causing the combined strip to bend away from the one which is expanding more. Often made into a snap action to create a switch which snaps open at a given temperature.
Binary	Binary is a number system using only the digits 0 and 1, well suited to computers as they can represent those digits by the on and off states of a switch.
BIOS	The BIOS or Basic Input/Output System was a program built in to PCs up until around 2011, which controlled the PC prior to booting and initiated the boot process. (Newer computers use UEFI). See BIOS vs UEFI in the Wiki.
Bipolar Transistor	A bipolar or junction transistor is a type of transistor with 3 semiconductor layers. See Transistors in the Wiki.
Bit	A bit or binary digit is the smallest unit of data, representing one of two values or states such as 1/0, or yes/no, or love Marmite/hate Marmite.
BJT	A bipolar or junction transistor is a type of transistor with 3 semiconductor layers. See Transistors in the Wiki.
Bloatware	Bloatware is the largely useless junk software, often time-limited free trials, bundled by a manufacturer with a Windows computer or an Android tablet or phone. The worst of it is commonly also referred to a crapware (not a term to be used in the most genteel circles). The term may also refer to regular software bloated with functions few uses will need.
Bluetooth	Bluetooth is a personal area networking protocol allowing an individual's personal devices to communicate with one another.
Boost regulator	A boost regulator is a type of switch-mode voltage regulator which produces a higher output voltage than the input.
Bootstrap	A bootstrap is a multi-stage process, for example for starting a computer, in which each stage (starting with a very simple one) enables the next more sophisticated stage to happen, like pulling yourself up by your bootstraps. A bootloader is a program which uses the bootstrap principle to "boot up" a computer into its running state.
Bot	A bot (short for Robot) is a computer which has been compromised in order to autonomously use its Internet connection for spreading spam or malware. Usually it will be one of many such computers comprising a "botnet", under the control of a "bot herder".
Brick	To "brick" a device is to render it completely useless, i.e. having as much functionality as a house brick. This can happen, for example, if the wrong type of firmware is accidentally loaded. "Unbricking" it can then be challenging, sometimes virtually impossible.
Bridge Rectifier	A bridge rectifier consists of four diodes connected in a diamond shape. Applying AC to two opposite corners, you get DC out of the other two. See Linear Power Supplies in the Wiki.
Brush	A brush in an AC/DC motor consists of a small block of graphite, a pair of which feed the current to the rotor via the commutator. See DC and Universal (AC/DC) Motors in the Wiki.
Brushless Motor	An Electronically Commutated or Brushless motor is a type of DC motor which uses an electronic circuit to switch the current between several coils in order to keep the rotor turning, instead of using a commutator and brushes to perform this function. See DC and Universal (AC/DC) Motors in the Wiki.
Buck regulator	A buck regulator is a type of switch-mode voltage regulator which produces a lower output voltage than the input.
Bus	A bus in computer terms is a method of allowing serveral modules to communicate. It may consist of any number of parallel wires to which all modules are connected. In some implementations one module will be the "master" and will control all the others, termed "slaves", whilst in others, any module may bid for control of the bus in order to become a temporary master.
Byte	A byte or octet is 8 bits, which can be used to represent a single letter, number or punctuation mark, or a number between 0 and 255, or anything else that can have up to 256 values.
C
Carrier	A carrier is a radio frequency signal which is modulated by an audio signal (for example) so that the audio can be transmitted by radio.
Capacitor	A capacitor or a condenser (an old-fashioned term) is an electronic component which can store electric charge. Its capacitance (or its capacity for storing charge) is measured in Farads. See Capacitors in the Wiki.
Capacitance	Capacitance is the property of storing electric charge measured in Farads, which is the purpose of a capacitor.
Cathode	A cathode is a negative electrode.
Ceramic resonator	A ceramic resonator is similar in function to a quartz crystal but using a piezoelectric ceramic such as lead zirconium titanate instead of quartz. They are used as a cheaper alternative to a quartz crystal where the precise frequency is not so critical, and also as the tuning filter in the IF stages of some radios.
Circuit	Electricity really hates piling up, so it will only flow if it can go all around a complete circuit and come back to where it started. (The usual symbol for an on/off switch is a circle broken by a dash, representing an interrupted circuit.)
Circuit Diagram	A vacuum cleaner, for example, may contain several paths or "circuits" around which the current can flow, and a computer may contain a huge number. A circuit diagram is a picture of all those paths (or some of them) allowing you to see how the parts are connected together and hopefully understand how it works.
CLI	A CLI or Command Line Interface is a type of user interface in which the user types commands at a prompt. Also known as a Character User Interface or CUI (pronounced "coo-ee"). The acronym is also used in telephony to mean Calling Line Identity, i.e. the phone number initiating a call.
Client	A client in computer terms is a computer (or a program running on a computer) the main purpose of which is to interact with a user. It relies on a server to do the bulk of the processing or data storage.
Cloud	On their network diagrams, computer network engineers frequently represent the Internet or unspecified remote computers by a fluffy cloud. The "cloud" has therefore come to be used to refer to any remote third party computer services or networks. Bear in mind though that the cloud is in reality just someone else's computer. It's worth asking yourself, therefore, how much you trust them with your data!
CMOS	CMOS (pronounced "sea-moss") or Complementary Metal Oxide Semiconductor is a class of very low power logic gates. In early PCs the BIOS settings were held in CMOS memory kept alive by a small backup battery, and the term is still sometimes (erroneously) used for this memory, although Flash memory is now universally used. (The backup battery now only keeps the real time clock alive.)
COB	COB or Chip On Board is a method of construction in which one or more bare semiconductor chips are mounted directly on a circuit board rather than encapsulated in individual packages. It is often used in mass produced items such as calculators in which the chip is covered by a blob of black resin, and more recently to mount many LED chips on a base to form a high power lamp, or on a thin strip to make a filament type of LED lamp.
Commutator	The commutator is the part of the rotor of an AC/DC motor which receives the current from the brushes. See DC and Universal (AC/DC) Motors in the Wiki.
Compact Florescent Lamp
Conductor	A conductor is a material which conducts electricity. Metals and carbon are conductors.
Continuity	Continuity refers to an intentional low resistance electrical path between two points in a circuit. A continuity tester is a device which tests for continuity, often giving an audible beep when continuity is detected.
CPU	The CPU or Central Processing Unit is the "brains" of a computer which executes a computer program.
Crapware	Crapware is a somewhat vulgar term (best not used in polite company) referring to the largely useless junk software often bundled by a manufacturer with a Windows computer or an Android tablet or phone. (In some circles you might prefer to use the more genteel term bloatware.)
Crimp	A crimp is a type of connection where a wire is inserted into a tubular receptacle which is then crushed onto the wire with a special purpose crimping tool. Correctly formed, a crimped connection is generally stronger than a soldered one, and can be done faster and more reliably in a production environment.
CRT	A CRT or Cathode Ray Tube is the image-producing device in old fashioned televisions, computer monitors, radar sets and oscilloscopes. It consists of an evacuated glass enclosure in which electrons are fired at a fluorescent screen to create the image. Superseded by much less bulky LCD displays.
Crystal	A crystal is most often taken to mean a quartz crystal, but may sometimes refer to the diode (these days germanium but originally a crystal of galena) in a crystal set.
Crystal set	A crystal set is the simplest form of radio. It needs no batteries or other power source apart from the energy provided by the radio waves it receives. Modern crystal sets use a germanium diode to recover the audio from the radio signal but the earliest (pre-war) examples used a crystal of galena, hence the name. See How Radios Work in the Wiki.
CSM	CSM or Compatibility Support Module is an option which may be present and enabled in a UEFI to support booting from legacy MBR-formatted disks. See UEFI and GPT.
CUI	A Character User Interface or CUI is a user interface operated exclusively with a keyboard by typing commands at a prompt or selecting options from a menu. This is in contrast to a Graphical User Interface. Also known as a Command Line Interface or CLI.
Custom ROM	A custom ROM is an alternative to the operating system provided by the vendor for a device such as a smartphone.
D
DAB	DAB or Digital Audio Broadcasting is the standard used for digital broadcast radio.
Daughter board	A daughter board is a subsidiary circuit board which plugs into a main circuit board, or motherboard.
DC	DC or Direct Current is an electrical supply which goes in one direction only. See AC and DC in the Wiki.
DDOS	A DDOS or Distributed Denial of Service attack is one launched simultaneously from many different computers across the Internet, so increasing its power and making it harder to defend against.
Decimal	Decimal is the number system we commonly use in everyday life, based on the digits 0 - 9.
Desktop Manager	The Desktop Manager is that part of an operating system which represents the computer's applications and tools on the screen as items on a desk.
Diac
Dielectric	A dielectric is an insulating material in which the positive and negative charges in the atoms or molecules can be displaced slightly in opposite directions from their natural positions by an electric field, causing the material as a whole to exhibit an electrical charge.
Digital	Digital is a method of representing, processing and transmitting a signal or a quantity as a number or sequence of numbers, usually expressed in binary as ones and zeros.
DIL	DIL or Dual In-Line is a type of IC packaging with 2 parallel rows of pins, designed for through-hole assembly.
Diode	A diode is a 2 terminal electronic component allowing a current to flow in one direction but not the other. See Diodes and Rectifiers in the Wiki.
Dipole	A dipole is a thing with two opposite ends, such as a magnet (with North and South poles), a molecule with a positive charge at one end and a negative charge at the other, or a type of aerial consisting of two rods placed end to end.
Distro	A distro is a flavour of Linux comprising a kernel, a desktop manager and a selection of utilities.
Demodulation	Demodulation is the process of recovering an audio or TV signal (for example) from the RF signal received from an aerial.
Denial of Service	A DOS or Denial of Service attack is an attack which renders a service (such as a website) unusable for its proper purposes. See also DDOS.
DLP	DLP or Digital Light Processing is a class of data projector using a Digital Micromirror Device to produce the image.
DMD	A DMD or Digital Micromirror Device is an imaging device in which the brightness of each pixel is determined by an individual tiny electrically operated mirror.
DMM	A DMM or Digital Multimeter is a Multimeter with digital readout.
DOA	DOA or Dead on Arrival refers to a brand new item which fails to work when first unboxed and switched on.
DOS	A DOS or Denial of Service attack is an attack which renders a service (such as a website) unusable for its proper purposes. See also DDOS.
DOS	MSDOS (Microsoft Disk Operating System, often simply referred to as DOS) was a Microsoft operating system predating Windows, and lacking a graphical user interface.
DPDT	A DPDT (Double Pole Double Throw) switch is just two SPDT (or two-way) switches ganged together and operated in tandem. It's used to simultaneously switch two separate circuits between alternate paths.
DPST	A DPST (Double Pole Single Throw) switch is just two SPST (simple on/off) switches ganged together and operated in tandem. It's often used to cut both the live and neutral from a mains supply for safety.
E
Earth Loop	When two pieces of audio equipment are connected together, each having its own earth connection, some of the small current that should be going from one of the pieces of equipment to its own earth connection instead goes via the connection to the other equipment's earth, causing an objectionable hum in the audio output. This is known as a Ground or Earth Loop.
E-Ink	E-Ink is a type of display, usually monochrome only and commonly used in e-book readers. It has the advantage of using very little power, and can be viewed by reflected ambient light rather than needing an active backlight.
ECC	ECC or Error Correcting Codes are additional bits added to a block of data, derived mathematically from the data and enabling errors arising during storage or transmission (up to a point) to be detected and corrected.
EHT	EHT or Extremely High Tension is the name given to the very high voltage supply (usually in excess of 1,000V) required by cathode ray tubes in old fashioned televisions, magnetrons in microwave ovens and a few related devices. Needless to say, it can be very dangerous!
Electricity	Electricity: tamed magic... well, mostly tamed.
Electrode	An electrode is an electrical conductor making contact with a non-metallic part of a device such as a semiconductor or the electrolyte in a battery or the vacuum in a valve.
Electrolyte	An electrolyte is a conducting liquid containing negative and positive ions, such as the liquid in a battery.
Electrolytic Capacitor	An electrolytic capacitor is a type of capacitor which employs an electro-chemical process to achieve a high value of capacitance. However, their failure rate is relatively high - see Capacitors in the Wiki.
Electromagnet	An electromagnet is a coil of wire wound around an iron core. A current passing through the coil magnetises the iron for only as long as the current flows. Typically used in electro-mechanical devices such as toasters, where it holds the lever down while the toast is toasting.
Electromagnetic Wave	An electromagnetic or EM wave is a kind of wave in space in which energy is continually exchanged between electric and magnetic fields similarly to how energy in a ripple on a pond is continually exchanged between gravity and momentum. Radio waves, light, x-rays and gamma rays are all EM waves of different wavelengths.
Electron	An electron is a sub-atomic particle carrying a negative electrical charge. A flow of electrons constitutes an electric current.
Electronically Commutated Motor	An Electronically Commutated or Brushless motor is a type of DC motor which uses an electronic circuit to switch the current between several coils in order to keep the rotor turning, instead of using a commutator and brushes to perform this function. See DC and Universal (AC/DC) Motors in the Wiki.
EMF	EMF or Electro-Motive Force is just a fancy name for Voltage, most often used in the context of a dynamo, or the dynamo effect in an electric motor.
Encryption	Encryption is the process of transforming a message or data (the "cleartext") into a completely unintelligible form (the "ciphertext"). Only with knowledge of a secret key can the ciphertext be transformed back into the cleartext.
Epoxy	Epoxy is a type of very strong adhesive which comes as a resin and a separate hardener. It starts to set only when the two are mixed together.
ES	ES or Edison Screw is a screw-fitting lamp base. Several sizes are in common use. See Types of bulb in the Wiki.
ESD	ESD or Electrostatic Discharge is static caused by friction such as shoes on a carpet, and particularly noticeable in conditions of low humidity. Even if not perceptible, it can damage sensitive electronic components.
ESP	ESP or EFI System Partition is a partition on a GPT-formatted disk containing system and boot information. See UEFI and GPT in the Wiki.
Ethernet	Ethernet is a family of networking protocols defining several methods by which packets of data can be transmitted across a computer network.
F
Farad	The Farad is the unit of capacitance. Named after British scientist and experimenter Michael Faraday, who made important discoveries in the fields of electromagnetism and electrochemistry. See Capacitors in the Wiki.
FAT	FAT or File Allocation Table is a type of filing system invented by Microsoft for use on DOS and earlier versions of Windows. Its several variants (FAT16, FAT32, ExFat) are supported by a very wide range of operating systems. It has the advantage of simplicity but is limited in the size of disks it can support and doesn't support file attributes such as file permissions.
Ferrite rod aerial	A ferrite rod aerial is a type of aerial used for medium and long wave radio reception and consisting of a rod or block of ferrite (a magnetic but insulating material) with one or more coils of wire wound around it. See How radios work in the wiki.
FET	A FET or field-effect transistor is a type of transistor where the current flowing between two of its terminals (the source and the drain) is controlled by an electric field created by a voltage on a third terminal (the gate). See Transistors in the Wiki.
FFC	An FFC or Flat Flexible Cable is a type of cable with parallel conducting tracks on a plastic ribbon. Used extensively for connections between separate modules in many types of device.
Filament Lamp	A filament lamp is a type of light bulb containing a thin tungsten wire which glows white hot when a current is passed through it. Such bulbs are highly inefficient and can no longer be legally sold. See Types of bulb in the Wiki.
Filing System	The filing system is that part of an operating system which manages a disk and presents it to the user as individual named files, hiding all the messy details about where on disk the data is actually stored.
Firewall	A firewall is a networking device which controls the traffic passing through it according to a defined policy, typically to prevent hackers from accessing a private network, or to block maliciously crafted requests.
Firmware	Firmware is the Software which is built in to the hardware of a computer or smart device and is expected to be rarely if ever changed.
Flash	Flash is a term used for a type of memory that can only be changed by erasing and rewriting a substantial block of cells at a time. Flashing is used as a verb for the process of rewriting or updating such a memory.
Flux	Flux is a liquid or paste which helps to remove oxide or other deposits in order to make good solder joints. Only usually needed for challenging soldering jobs such as surface mount as solder normally contains a core of flux.
FM	FM or Frequency Modulation is a method of modulation in which the frequency of an RF signal is modulated by the AF signal to be transmitted. See Frequency Modulation in the Wiki.
FPC	An FPC or Flexible Printed Circuit. A flexible substrate with printed circuit tracks, generally brown or dark orange, and often having surface mount components or connectors soldered directly onto it.
FPGA	A Field-Programmable Gate Array is an integrated circuit containing a large number of uncommitted simple circuit elements such as logic gates. All the interconnections can be programmed after manufacture to create a complex subsystem of the designer's choosing.
Frequency	Frequency is the number of complete (positive and negative) cycles of AC which occur per second. Measured in Hertz.
Full wave rectifier	A full wave rectifier is one that which uses both the positive and negative half cycles of the AC input to produce a DC output. Most often this is achieved with a bridge rectifier.
Fuse	A fuse is a thin piece of low melting point wire designed to melt and so break the circuit if too much current flows due to a fault.
G
Glarbage collection	Garbage collection is the process of collecting up disused memory or storage to allow it to be reused. In an SSD it refers to the internal process by which it consolidates still needed data scattered amongst disused data in order to be able to erase complete data blocks for reuse.
Germanium	Germanium is a semiconductor used for the manufacture of early transistors. It has been superseded by silicon for nearly all purposes but is still used in conjunction with other materials in some applications such as solar cells and LEDs.
GB	A GB or Gigabyte is 1000 megabytes, but the term is often used for 1024 megabytes, more properly written GiB.
GHz	A GHz is a billion (thousand million) Hz
GiB	A GiB is 1,024MiB, or 1,073,741,825 bytes. It is as much as can be addressed with 30 bits.
GPT	GPT or GUID Partition Table is the name given to a newer method of organising the contents of a disk. See MBR vs GPT disk layout in the wiki.
Grid	The grid is the control electrode in a valve. Driving it negative with respect to the cathode restricts or prevents a flow of electrons from the cathode to the anode.
Ground Loop	When two pieces of audio equipment are connected together, each having its own earth connection, some of the small current that should be going from one of the pieces of equipment to its own earth connection instead goes via the connection to the other equipment's earth, causing an objectionable hum in the audio output. This is known as a Ground or Earth Loop.
GUI	A GUI (pronounced "goo-ey") or Graphical User Interface is a user interface operated principally using a graphical display and a mouse or other pointing device, as opposed to a Character User Interface, operated just by a keyboard and text on a display.
GUID	A GUID or Globally Unique Identifier is a 128 bit number used for identifying various types of resource in a computer system. A 128 bit number is so large that if you choose one at random, there is practically no chance that anyone else will ever pick the same one, even if you were to wait billions of years.
H
Half wave rectifier	A half wave rectifier is one which uses only the positive (or the negative) half cycles of the AC input to produce a DC output, by using a single diode.
Hall Effect	When a current flows through a conductor and a magnetic field is applied to the conductor at right angles, the current tends to be pushed to one side of the conductor, at right angles to the magnetic field. This is the Hall Effect. Used in a Hall effect device to detect or measure a magnetic field, for example in smartphones to detect the earth's magnetic field to implement a compass.
Halogen Lamp	A halogen lamp is an improved form of filament lamp with longer life and better efficiency, but still falling far short of compact fluorescent and LED lamps. See Types of bulb in the Wiki.
Hard Disk	A hard disk or HD is a common type of computer main long term storage in which data is held on a rapidly spinning magnetic disk. (Increasingly, modern computers use a Solid State Disk instead.)
Hardware	Hardware is the physical electronic and electromechanical parts which comprise a computer.
HD	A hard disk or HD is a common type of computer main long term storage in which data is held on a rapidly spinning magnetic disk. (Increasingly, modern computers use a Solid State Disk instead.)
Heat Sink	A heat sink is a piece of metal designed to conduct heat away from a component so as to prevent it getting too hot. When soldering, a pair of pliers can be applied to a lead of a sensitive electronic component as a heat sink to prevent the heat of the solder from damaging the component.
Henry	The Henry is the unit of inductance. Named after American scientist Joseph Henry, who made important discoveries in the field of electromagnetism (independently of Michael Faraday). See Inductors in the Wiki.
Hertz	The Hertz (Hz) is the unit of frequency, numerically equal to cycles per second. Named after German physicist Heinrich Hertz, who was the first to prove the existence of previously theorised electromagnetic waves.
Hexadecimal	Hexadecimal is a number system based on 16 digits which are represented by 0-9 and A - F. This is useful because each hexadecimal digit can represent 4 bits, and 2 hexadecimal digits can represent 8 bits or a byte.
Hole	A hole is the lack of an electron in a crystal lattice where there should be one. This leaves behind a positive charge. An electron from an adjacent atom can move into the hole causing the hole to move to the next atom. In this way, holes can act like positively charged particles and carry an electric current.
HT	HT or High Tension is the name given to the main high voltage supply required by valve equipment, usually in the region of several hundred volts. Warning: this can be dangerous!
Hum	Hum is a constant low frequency sound in the output of audio equipment due (for example) to inadequate smoothing in the power supply and residual ripple in its output, or to the presence of a ground loop. Alternatively, it's what you do when you don't know the words.
Hz	The Hertz (Hz) is the unit of frequency, numerically equal to cycles per second. Named after German physicist Heinrich Hertz, who was the first to prove the existence of previously theorised electromagnetic waves.
I
IC	An Integrated Circuit or IC is a circuit comprising anything from dozens to billions of electronic components, all fabricated on a single silicon chip. See Integrated Circuits in the Wiki.
IF	IF or Intermediate Frequency is a frequency between the RF and AF in a superhet radio at which most of the amplification and selection of the required station occurs.
IGBT	An Insulated Gate Bipolar Transistor or IGBT is a device in which a MOSFET and a bipolar transistor are integrated into a single structure offering many of the advantages of both types of device. IGBTs are therefore capable of operating at both very high voltages and very high currents.
Impedance	For most practical purposes, impedance is the same as resistance, and also measured in ohms. Resistance is the number of steady volts you need to push a steady amp through a circuit, but if the voltage is AC or is varying, then inductance and capacitance add a kind of momentum and springiness to the way the current responds to the voltage. The volts per amp is then likely to be different, and is termed impedance.
Inductance	Inductance is the property of a coil of wire (or even a straight piece of wire) of storing energy in the form of a magnetic field generated by a current flowing through it. It is measured in Henrys. When the voltage driving it is reduced or withdrawn, this energy is released back into the current, giving it a kind of momentum and tending to keep it flowing.
Induction motor	An induction motor is a type of electric motor which runs only on AC. Often used in fans. See Induction Motors in the Wiki.
Inductor	An inductor (or choke) is a coil of wire, very often wound around a magnetic core, providing a kind of momentum to the flow of electricity. Its inductance is measured in Henrys. It is commonly known as a choke when used to block high frequencies or reduce ripple from the AC mains. See Inductors in the Wiki.
Inrush	Inrush is a momentary surge of current drawn by a device on first switching it on, often causing a noticeable dimming of lights for a fraction of a second. Some equipment is fitted with a slow blow fuse to withstand the inrush. Items containing a large electric motor often have a significant inrush current as the motor starts.
Insulation displacement	Insulation displacement is a type of connection where insulated wire, without stripping the insulation, is forced into a slot with sharp inner edges. These edges cut through the insulation and bite into the wire. Commonly used on telephone and network connectors, and connectors designed for attachment to ribbon cables.
Insulator	An insulator is a material which doesn't conduct electricity. Examples are plastic, wood, glass and fresh air.
Integrated Circuit	An Integrated Circuit or IC is a circuit comprising anything from dozens to billions of electronic components, all fabricated on a single silicon chip. See Integrated Circuits in the Wiki.
Interface	An interface in computer terms is a connection between two (or more) computers or components of a single computer, or the means by which a computer interacts with the human users. In the former case it will be defined by the types of plug and socket used, and a specification of the signals passed across and their meanings.
International Octal	IO or International Octal was a type of 8-pin valve base with a keyed spigot, used on older and larger valves, and often referred to simply as "octal".
Internet	The Internet is a global computer network in which (unlike the telephone network) information is transmitted as relatively small packets of data, each packet individually addressed and individually routed to its destination. Many different services use this foundation, two of the best known being the World Wide Web (which delivers web pages) and email.
Internet of Things	The Internet of Things (or IoT) refers to the networking of smart devices which have not traditionally been thought of as computers, such as vehicles, security cameras, home heating systems, kitchen appliances and medical devices, as well as industrial machines and sensors. As a result, they can be remotely monitored and controlled or can feed data back to a service provider. However, if they are not properly secured (as is often the case) they can represent a serious security risk.
Intranet	An intranet is a computer network using the same technology as the Internet but existing only within the confines of a particular company or organisation. However, most intranets have a gateway through which users can access the wider Internet, usually subject to certain restrictions.
Ion	An ion is a positively or negatively charged atom.
IoT	The Internet of Things (or IoT) refers to the networking of smart devices which have not traditionally been thought of as computers, such as vehicles, security cameras, home heating systems, kitchen appliances and medical devices, as well as industrial machines and sensors. As a result, they can be remotely monitored and controlled or can feed data back to a service provider. However, if they are not properly secured (as is often the case) they can represent a serious security risk.
IP	IP or Internet Protocol is a set of standards defining how data (whether it be it web pages, emails or anything else), broken up into individually addressed packets, is moved across the Internet from a source to a destination. There are two versions in use: predominantly Version 4 (IPv4), and also version 6 (IPv6), which allows vastly more addresses.
ISO	ISO is the International Organisation for Standardisation which publishes thousands of standards, but in computing the term is often used to refer to one particular standard for the layout of information on a CD or DVD. A Linux or Windows installation file, for example, will often be available as an ISO file, which is a byte-for-byte disk image which can be burned directly to a DVD.
J
Jack	A jack is the type of plug (and its socket) familiar as a headphone plug.
jFET	A Junction Field Effect Transistor is a type of field effect transistor where the gate is formed by a semiconductor junction. See Transistors in the Wiki.
Joule	The joule is the unit of energy, being the amount of work done by 1 watt in 1 second. 4.2 Joules are equivalent to 1 calorie of heat. The unit is named after English physicist James Prescott Joule, best known for discovering the law which states how much heat is produced in a conductor by the passage of an electric current.
Junction Transistor	A bipolar or junction transistor is a type of transistor with 3 semiconductor layers. See Transistors in the Wiki.
K
kB	A Kilobyte (kB) is 1024 bytes.
Kernel	The core of an operating system, which concerns itself with managing the hardware resources at the lowest level. It normally has no user interface, this being provided by another program or programs referred to as the shell. Think of a nut. The shell is what you see but the goodness is inside in the kernel.
Keylogger	A keylogger is a type of malware or a hardware device which logs your keystrokes (including passwords, credit card numbers etc.) and sends them back to a malicious party, or stores them for later collection.
kHz	A kHz is 1,000 Hz
L
LAN	A Local Area Network is a computer network covering a limited area such as a single building, or at most, a campus.
LCD	A Liquid Crystal Display is the type of display used in most calculators, phones, tablets and computer monitors. It uses a special liquid which acts on polarised light under an electric field.
LDR	A Light Dependant Resistor is a device having a high resistance in the dark but whose resistance falls with increasing illumination.
LED	A LED is a Light Emitting Diode, similar to any other semiconductor diode but specially crafted so that photons are emitted as electrons fall into holes at the junction between a p-type and an n-type region.
Linear Regulator	A linear regulator is a circuit often in the form of an IC which produces a constant well defined voltage from a variable or poorly defined higher voltage input, simply by absorbing excess voltage and turning it into heat. See Linear Power Supplies in the Wiki.
Linux	Linux is a free, open source operating system kernel which, packaged into a distro, can be used as an alternative to Windows or OS/X.
Live Linux	A live linux is a Linux system on a bootable CD/DVD or memory stick which can be run from that media without affecting the installed operating system on the hard disk.
Li-ion	Lithium-ion and lithium-polymer are types of high capacity rechargeable battery much used in portable electronic devices and electric vehicles. See Batteries in the Wiki.
Li-polymer	Lithium-ion and lithium-polymer are types of high capacity rechargeable battery much used in portable electronic devices and electric vehicles. See Batteries in the Wiki.
Load cell	A load cell is a device for measuring a force and presenting it as an electrical signal. See Digital weighing scales in the wiki.
Loupe	A loupe is a magnifying glass, generally without a handle. Jewellers and watchmakers commonly use an eye loupe, which with a little practice can be held in the eye socket, leaving both hands free.
LT	LT or Low Tension is the name given to the low voltage supply required by valve equipment to heat the cathodes of the valves. A common value is 6.3V.
M
M.2	M.2 is a type of expansion card commonly (but not exclusively) used for SSDs, especially in ultra-thin laptops where a conventional 2.5" SSD would be too bulky. There are several types distinguished by the position of one or more keyways in the edge connector. See All about SSDs in the Wiki.
Magnetron	A magnetron is the device which produces the microwaves in a microwave oven, originally developed during WWII as a small and high powered source of microwaves for radar applications. A strong magnet causes a stream of electrons to circle around a series of cavities causing them to resonate electrically in much the same way as blowing across the top of a bottle can make it resonate.
Malware	Malware is a term used for any bad piece of software which attempts to get onto your computer for malicious purposes.
MB	A MB or megabyte is 1000 kilobytes, but the term is often used for 1024 kilobytes, which is more properly written MiB.
MBR	The MBR or Master Boot Record is the name given to an older method of organising the contents of a disk. See MBR vs GPT disk layout in the wiki.
MEMS	MEMS or MicroElectroMechanical Systems are devices containing microscopic moving parts and fabricated using the techniques developed by the microelectronics industry for manufacturing integrated circuits. They find many common applications, including the accelerometers and microphones in smartphones, pressure sensors, the print head in some inkjet printers and the Digtal Micromirror Device which creates the image in some data projectors.
Metal Halide Lamp	A metal halide lamp is a very efficient type of lamp which operates by creating an electrical discharge through a mixture of gasses such as mercury vapour and a metal iodide or bromide (the "halide") in order to give the full spectrum of white light. They are not used domestically except as the light source in a data projector as they can take several minutes to reach full brightness, run very hot, and can explode if abused. They are frequently used for commercial lighting (e.g. in shops) and for floodlighting.
MHz	A MHz is a million Hz
MiB	A MiB is 1,048,576 bytes, the amount that can be addressed with 20 bits.
MicroSATA	MicroSATA or μSATA is a smaller version of the standard SATA connector used on modern 2.5" and 3.5" disk drives, and used on smaller 1.8" drives.
Microswitch	A microswitch is a type of switch often found in electromechanical devices, typically actuated by a safety interlock, a cam or a lever rather than directly by the user. They generally come in a few standard sizes but rather more configurations of actuating lever or button and arrangements of the contacts.
MiniSATA	mSATA (or mini SATA) is a type of expansion card commonly (but not exclusively) used for SSDs, especially in ultra-thin laptops where a conventional 2.5" SSD would be too bulky. See All about SSDs in the Wiki.
Modem	A modem or "modulator/demodulator" is a device used to perform 2-way conversion of a data stream between digital and analogue forms, for example to communicate data over a voice or broadband (analogue) connection.
Modulation	Modulation is the process of impressing an audio or TV signal (for example) onto an RF signal for transmission. See How radios work in the wiki.
MOSFET	A MOSFET or Metal Oxide Field Effect Transistor is a type of field effect transistor where the gate is formed by metal deposited on an extremely thin insulating oxide layer. See Transistors in the Wiki.
Motherboard	The motherboard in a computer (or other piece of electronic equipment) is the main circuit board into which one or more daughter boards can be plugged.
MOV	An MOV or Metal Oxide Varistor is the commonest type of Varistor.
mSATA	mSATA (or mini SATA) is a type of expansion card commonly (but not exclusively) used for SSDs, especially in ultra-thin laptops where a conventional 2.5" SSD would be too bulky. See All about SSDs in the Wiki.
MSDOS	MSDOS (Microsoft Disk Operating System, often simply referred to as DOS) was a Microsoft operating system predating Windows, and lacking a graphical user interface.
MTBF	MTBF or Mean Time Between Failures is a measure of the reliability of an item, most usually measured in hours (or thousands of hours).
Multimeter	A multimeter is an electrical test instrument which measures voltage, current, resistance and often several other things. Most usually they have a digital readout but analogue ones with a dial are also available. See How to use a multimeter in the Wiki.
N
N-type	N-type is the name given to a Semiconductor material doped with an impurity whose atoms have one too many electrons to match the crystal lattice. These "spare" electrons can carry an electric current as negative charges.
Neon	Neon is an inert gas present in trace quantities in dry air, but in electronics a "neon" generally refers to a gas discharge lamp giving a characteristic orange glow and much used as an indicator lamp before the advent of LEDs and in signage. Other gasses can be used to give different colours but the lamps using them are still generally referred to as "neons".
NiCd	NiCd or Nickel Cadmium is a type of rechargeable battery, similar in many ways to Nickel Metal Hydride but with higher maximum charge and discharge rates making it suitable for power tools and radio controlled models, and having greater tolerance to abuse, but suffering from a "memory effect" and containing toxic cadmium. See Batteries in the Wiki.
NiMH	NiMH or Nickel Metal Hydride is a type of rechargeable battery, similar in many ways to Nickel Cadmium but with greater capacity, reduced "memory effect" and containing no toxic cadmium, but having a higher self-discharge rate. See Batteries in the Wiki.
NPN	A type of junction transistor comprising a P-type layer sandwiched between two N-type layers.
NTC Resistor	A negative temperature coefficient or NTC resistor is a resistor whose resistance falls strongly with increasing temperature, unlike most resistors whose resistance increases slightly with temperature. Used as a temperature sensing component in electronic thermometers, to warn of overheating, particularly in lithium batteries, and as an inrush current limiter.
NVMe	NVMe (Non-Volatile Memory Express) is an interface standard and logical access method optimised for accessing SSDs. PCIe is commonly used as the physical interface.
O
Object Code	Software in a form that can be directly executed by a computer, but is virtually unintelligible to humans, simply comprising binary numbers.
Octal	A number system based on only 8 digits 0 - 7. This is useful because each octal digit can represent 3 bits. See also International Octal (a type of valve base).
Octet	A byte or octet is 8 bits, which can be used to represent a single letter, number or punctuation mark, or a number between 0 and 255, or anything else that can have up to 256 values.
OEM	Original Equipment Manufacturer. A somewhat confusing term generally meaning a company which sells another company's manufactured goods under its own branding and with its own warranty. Also used of software, for a licence allowing a computer manufacturer to install the software on systems they build. (In fact, the manufacturer could be you, building a computer from standard parts.)
Ohm	A bit like friction, the Ohm is the unit of resistance to flow of electricity. Named after German mathematician and physicist Georg Simon Ohm, chiefly remembered for discovering Ohm's Law. See Electric circuits, volts amps watts and ohms in the Wiki.
Ohm's Law	The amount of current (Amps) flowing in a circuit is the pressure (Volts) divided by the resistance (Ohms), or I = V / R.
OLED	Organic Light Emitting Diode. A light emitting diode using an organic (i.e. carbon-based chemical) semiconductor rather than one based on inorganic materials such as silicon or a compound of gallium. Also used to refer to a display made up of many OLEDs.
Open Circuit	Electricity likes to go all the way round a circuit and back to where it started. When there is a break in the circuit preventing the current from flowing (maybe due to a fault) this is termed an open circuit.
Open Source	Software or hardware the source code or design of which is non-proprietary and made freely available for anyone to inspect or modify for their own purposes.
Operating System	The software which manages the computing, storage and communications resources of a computer, and presents a user interface allowing the user to run and interact with programs as desired.
Operational Amplifier	An operational amplifier (commonly known as an op amp) is a high gain amplifier, usually an integrated circuit. With a feedback loop it can be used to construct a variety of circuit functions such as high quality amplifiers and filters.
Opto coupler	An LED and a phototransistor, electrically isolated but in a single packaged. Used for signalling between two circuits which must have no electrical connection between them.
Oscillator	A circuit which produces an AC output at a chosen frequency.
Oscilloscope	An oscilloscope (often referred to simply as a "scope") is a device for producing a graphical representation of how an electrical signal varies with time.
Own	In hacker slang, to gain full control of a device or computer through subversive means. See also Pwn.
P
P-type	P-type is the name given to a Semiconductor material doped with an impurity whose atoms have one too few electrons to match the crystal lattice. These result in holes which can effectively move about and carry an electric current as positive charges. Think of bubbles in a liquid. These "holes" in the liquid effectively have a negative weight and so tend to rise against gravity.
PAN	A PAN is a Personal Area Network, i.e. a computer network linking a single person's devices, typically covering, at most, a single room.
Parallel	Two elements of a circuit are said to be in parallel if they are connected so that the current can equally pass through one or the other. For example, if you connect a battery to a bulb through two switches in parallel, the bulb will light if either or both switches are switched on.
Parallel Interface	A parallel interface is a data connection in which some number of bits (for example the 8 bits or a byte, or maybe 16 or 32 bits) are all sent simultaneously down separate wires. Whilst this can be a fast method of transmission, the speed and physical length of the connection are limited by the fact that slight differences in the path lengths may mean that not all bits arrive quite together.
Partition	A partition is a subdivision of a disk. Under the control of the operating system, a disk can be sliced up like a cake into several such partitions, each being dedicated to a particular purpose. Otherwise, you would have to install another physical disk for each purpose. Also, large computers are sometimes made to pretend to be several independent smaller ones, and these can also be referred to as partitions.
Password Manager	A password manager is a program or app which stores all your passwords in an encrypted vault under a (hopefully) strong master password. Many password managers will also generate and remember totally random unguessable passwords, and automatically apply them to login password prompts.
PAT Test	A PAT or Portable Appliance Test is a basic electrical safety test which should be applied to all mains operated electrical equipment before and after attempting a repair. It comprises a visual inspection and an automated test. See PAT testing in the Wiki.
PATA	PATA or Parallel ATA is an old parallel interface used for connecting storage devices to a computer. It has been superseded by SATA but may still be seen in old computers up to the early years of the millennium.
PCB	A PCB or printed circuit board is a board on which conductive copper tracks are used to make the connections between the components, rather than individual wires.
PCI	PCI or Peripheral Component Interconnect refers to a type of parallel computer bus and the expansion ports attached to it common in desktop computers in the late 90's and early 2000's. It has been superseded by PCIe and the original PCI has been renamed Conventional PCI.
PCIe	PCIe or PCI Express is a serial computer bus superseding conventional PCI and used for connecting peripheral devices of any kind to a computer.
PD	PD or Potential Difference is a difference of voltage between two points in a circuit.
Peltier	The Peltier effect is a thermo-electric effect by which a junction between two different conductors or semiconductors warms if a current is passed through it in one direction and cools if it is passed in the opposite direction. The converse of the Seebeck effect. Used for cooling in mini fridges.
Pentode	A pentode is a valve with 5 electrodes: an anode, a cathode and 3 grids: a control grid (as in a triode), a screen grid to reduce stray interaction between the control grid and the anode, and a suppressor grid which ensures that electrons hitting the anode stay there.
Phase-locked loop	A phase locked loop or PLL is a circuit in which both the frequency and phase of a variable frequency oscillator is locked to a reference frequency or to a frequency with a simple numerical relationship to it. The many applications include digital tuning in a radio and the generation of the different clock frequencies for the processor and RAM in a computer.
Phillips	Phillips is a type of cross-head screw or screwdriver, similar to but not quite the same as Pozidriv. See Types of screw head and screwdriver in the Wiki.
Phishing	Phishing is the criminal practice of sending emails or messages through social media inviting the recipient to open a malicious attachment or visit a malicious website.
Photon	A photon is a particle of light. Light has both wave-like and particle-like properties as described by Quantum Mechanics. If you don't understand that, don't worry. Nobody understands Quantum Mechanics.
Phototransistor	A phototransistor is a light-sensitive transistor used for sensing light.
Piezo	The piezo-electric effect is the property of some materials to develop a voltage across two opposite faces when a physical force is applied, or to stretch or squeeze slightly if a voltage is applied. Pronounced "pee-ayzo" by some and "pee-zo" by others. The Oxford Dictionary allows either, but wars have been started over less contentious issues. Used in some microphones and earpieces, quartz crystal frequency standards, and gas lighters.
Pinout	The pinout is a term used for the definition (as a list or as a diagram) of the connections to each of the pins of a plug, a socket or an electronic component such as an integrated circuit with multiple connections.
PIR	A PIR or Passive Infrared detector is a device which detects heat radiation from warm bodies. Often used to turn on an outdoor security light, or to allow a room light to be turned off automatically when no one is there.
Pixel	A pixel is an individual picture element, or coloured (or black and white) dot, thousands or millions of which make up a digital image.
PLL	A phase locked loop or PLL is a circuit in which both the frequency and phase of a variable frequency oscillator is locked to a reference frequency or to a frequency with a simple numerical relationship to it. The many applications include digital tuning in a radio and the generation of the different clock frequencies for the processor and RAM in a computer.
PNP	PNP is a type of junction transistor comprising a thin N-type layer sandwiched between two P-type layers.
Pot	A potentiometer (commonly known as a pot) is a resistor with a slider that can be moved along its length from one end to the other to tap off any required amount of resistance, for example for a volume control. See Potentiometer in the Wiki.
Potentiometer	A potentiometer (commonly known as a pot) is a resistor with a slider that can be moved along its length from one end to the other to tap off any required amount of resistance, for example for a volume control. See Potentiometer in the Wiki.
Power Supply	A PSU or Power Supply Unit is that part of a piece of equipment which takes the raw input power, for example from a battery or from the mains, conditions it, and delivers it at the voltage(s) the equipment needs internally.
Pozidriv	Pozidriv is a type of cross-head screw or screwdriver, similar to but not quite the same as Phillips. See Types of screw head and screwdriver in the Wiki.
PPE	PPE or Personal Protective Equipment is equipment such as safety glasses, dust and fumes extractors, respirators, ear plugs, protective clothing etc. used for protection against health and safety hazards.
Primary Battery	A primary battery is one which cannot be recharged but has to be thrown away (or recycled) once the chemicals it contains are exhausted.
PSU	A PSU or Power Supply Unit is that part of a piece of equipment which takes the raw input power, for example from a battery or from the mains, conditions it, and delivers it at the voltage(s) the equipment needs internally.
PrivEsc	An Escalation of Privilege (or EoP or PrivEsc) vulnerability is a software flaw which allows an attacker, already operating on your computer, to increase the privileges allowed to him so as to gain full control.
Privilege Escalation	An Escalation of Privilege (or EoP or PrivEsc) vulnerability is a software flaw which allows an attacker, already operating on your computer, to increase the privileges allowed to him so as to gain full control.
Protocol	A protocol in computing terms is a set of rules or standards defining the way in which data is to be exchanged between two or more computers or components of a single computer, for example what messages can be sent and in what contexts by each party, the permissible responses, and how those messages and responses are to be structured and interpreted. Think of it as a Highway Code for computers.
PUP	A PUP or Potentially Unwanted Program is any useless but generally (though not necessarily) harmless program, often installed by the computer vendor or (knowingly or unknowingly) installed along with downloaded software. The main purpose of it is usually to sell you some other software which you probably don't want.
PV Cell	A PV or Photo Voltaic Cell (commonly known as a solar cell) is a device for converting light into electricity.
Pwn	In hacker slang, to pwn (pronounced "pone") a device or computer is to hack and comprehensively take control of or "own" it. The term derives from a mis-typing of "own" in a video game in the 1990's.
Q
Quartz crystal	A quartz crystal is an electronic component containing a piece of quartz cut and polished so as to resonate at a very specific frequency. Usually housed in a silvery metal can, it's very easy to spot on a circuit board. See Crystals and Ceramic Resonators in the Wiki.
Qubit	A qubit is a quantum bit which can be in two different states simultaneously. We have never seen one of these at a Restart Party, and if we ever do we will probably be undecided about what to do with it.
R
RAM	RAM or Random Access Memory is the very fast memory used by a computer as working store for the programs and data currently being processed.
Ransomware	Ransomware is a particularly nasty form of Malware which encrypts all your files then demands a ransom in order to recover them for you. But there is no guarantee that you'll get you're data back even if you pay the ransom.
RAT	A RAT or Remote Access Trojan is a type of Malware which allows a remote attacker to see your screen, to move and click your mouse and to enter keystrokes on your keyboard, just as if he were sitting in your chair.
RCE	A Remote Code execution or RCE vulnerability is a software flaw which allows a remote attacker (probably somewhere else on the Internet) to execute arbitrary code of his choosing on your computer, usually as the first step in taking it over.
Remote Code Execution	A Remote Code execution or RCE vulnerability is a software flaw which allows a remote attacker (probably somewhere else on the Internet) to execute arbitrary code of his choosing on your computer, usually as the first step in taking it over.
Rectifier	A rectifier consists of a diode or 4 diodes connected together to convert AC to DC.
Reed switch	A reed switch is a sealed glass tube containing two magnetic contacts forming a switch. The switch can be closed by bringing a magnet near or by energising an electromagnet. It can be used as the switch in a reed relay or with a magnet attached to a moving part, for example to detect when a cover is closed, a printer paper drawer is fully inserted, or a float in a tank is indicating that the tank is full.
Relay	A relay is a device consisting of a switch or a bank of switches operated by an electromagnet. The switch(es) may be simple switch contacts or a reed switch.
Reservoir Capacitor	A reservoir or smoothing capacitor is a large value capacitor used for smoothing rectified AC. See power supplies in the wiki.
Resistance	Resistance is the property of resisting the flow of an electric current, much like how mechanical friction resists motion. It is measured in Ohms.
Resistor	A resistor is an electronic component generally containing a thin metal or oxide film or a thin wire through which electricity flows with difficulty. It's resistance is measured in Ohms. A resistor always generates heat, even if only a tiny amount, and any replacement must be rated in Watts accordingly. See Resistors in the Wiki.
RF	RF or Radio Frequency is a high frequency capable of being transmitted as a radio signal.
Ripple	Ripple is the name given to the residual variation after smoothing in the output voltage of a DC power supply. It occurs at the frequency or twice the frequency of the AC mains input.
RJ11	RJ11 is a type of connector commonly used with telephones. It has up to 4 connections and a kind of barb which prevents it from being accidentally pulled out of a matching socket.
RJ45	RJ45 is a type of connector commonly used on network cables. It has 8 connections and a kind of barb which prevents it from being accidentally pulled out of a matching socket.
RMS	Since AC is continually varying it can't be measured as a unique voltage. The RMS or Root Mean Square value (a mathematical construct) is the DC voltage that would deliver the same power into a given load.
ROM	ROM or Read-only memory is memory which can be read as often as you like but cannot (or cannot so easily) be changed. It is typically used for the firmware of a device.
Rootkit	A rootkit is an especially pernicious type of malware which masks its presence and hinders its removal by subverting the operating system in such a way as to expunge all references to itself and associated malware in listings of files and running programs.
Router	A router is a network device connecting two or more networks. It forwards packets of data received from one network to its destination or to another router closer to its destination on another network.
S
Sandbox	A sandbox is a kind of software "padded cell" in which untested or untrusted software can be tried out without risk to the host computer, a bit like a sandbox in which a child can play safely with the sand.
SATA	SATA or Serial ATA is a serial interface used for connecting storage devices to a computer. It supersedes the older PATA.
Schematic	A schematic is another name for a Circuit Diagram.
SCR	A thyristor, SCR or Silicon Controlled Rectifier is an electronic component with 3 leads used as a switch. See Transistors in the Wiki.
Secondary Battery	A secondary battery is one which can be recharged, restoring the chemicals it contains to their original state (or nearly).
Sector	A sector is the smallest block of data a disk can read or write. On older disks this was normally 512 bytes but in modern disks and SSDs it is usually 4096 bytes.
Semiconductor	A semiconductor is a material such as silicon, germanium or gallium arsenide which can be made to conduct electricity either by negatively charged electrons (N-type) or by positively charged electron vacancies, or holes (P-type), by adding small amounts of carefully chosen impurities.
Series	Two elements of a circuit are said to be in series if they are connected so that the current has to pass through one then the other. For example, if you connect a battery to a bulb through two switches in series, the bulb will only light if both switches are switched on.
SES	SES or Small Edison Screw is a smaller version of the ES screw-fitting lamp base. See Types of bulb in the Wiki.
Seebeck	The Seebeck effect is a thermo-electric effect by which a voltage is generated by a junction between two different conductors or semiconductors if it is heated or cooled relative to the rest of the circuit. The converse of the Peltier effect. Used by space craft venturing to the outer reaches of the solar system to generate electricity from a radioactive heat source such as a lump of plutonium.
Serial Interface	A serial interface is a data connection in which the bits are all sent one after another down the same wire. Consequently they can be sent very fast indeed and use simple connectors with few pins. The COM ports found on old computers, USB on modern ones and SATA are all examples of serial interfaces.
Server	A server in computer terms is a computer (or a program running on a computer) which offers storage or processing services to a client computer (or many such), the latter dealing with user interaction.
Shell	A shell is a program which offers a user interface to lower level parts of an operating system (referred to as the kernel), which themselves lack a user interface. The Mac or Linux command prompt is referred to as the shell, and the Windows Desktop is Windows' shell.
Short circuit	If two wires are touching which shouldn't the current may be able to take a short cut, meeting very little resistance and so allowing too much to flow. This is a Bad Thing and can cause your gadget to get hot or even catch fire. A short circuit can also be caused by an inadvertent solder bridge between two tracks on a printed circuit board, and this may simply cause the device to malfunction.
Shottky diode	A Shottky diode is a type of diode using a metal-semiconductor junction, mainly used in power supplies on account of its ability to operate efficiently at higher frequencies. See switching mode power supplies in the wiki.
SIL	SIL or Single In-Line is a type of packaging used for circuit sub-assemblies, resistor networks and some ICs, and having a single row of pins designed for through-hole assembly.
Sine wave	A sine wave is the mathematically simplest form of vibration or oscillation, which can be represented by a single frequency only with no harmonics or overtones. Exemplified by the motion of a pendulum swinging through a small angle, or the height of a point on a wheel above or below the axle as the wheel turns at constant speed.
S.M.A.R.T	S.M.A.R.T or Self-Monitoring, Analysis, and Reporting Technology is a technology built into all modern hard disks and solid state disks by which the device can monitor things like its error rate and report back, hopefully giving early warning of impending failure.
Smoothing	Smoothing is the process of turning raw rectified AC (which drops to zero twice in every cycle of the AC) into a reasonably constant voltage. See Why is a power supply needed? in the Wiki.
Snubber	A snubber is a circuit element designed to absorb the voltage spike which tends to be created when the current is abruptly switched off through an inductive load such as an electric motor, so protecting other parts of the circuit or other devices powered by the same source.
SoC	An SoC or System-on-Chip is a complex integrated circuit comprised of a number of standard building blocks making up a complete self-contained system on a single silicon chip. These building blocks would typically include one or more CPU cores, RAM, ROM and peripheral and interface blocks, perhaps including blocks implementing a display interface, WiFi or Bluetooth.
Software	Software is the step-by-step instructions which tell a computer what to do and how to do it.
Solar Cell	A PV or Photo Voltaic Cell (commonly known as a solar cell) is a device for converting light into electricity.
Solder	Solder is a relatively low melting point mixture of (most often) tin and copper or tin and lead, making it very convenient for making electrical connections. (Americans call it "sodder", and many electrons have been harmed in discussions as to which is correct. But the Americans are on their own on this one.)
Solenoid	A solenoid is an inductor or electromagnet in cylindrical form.
Solid State Disk	A Solid State Disk (or SSD) is a faster (though more expensive) alternative to a hard disk for a computer's long term storage. Instead of a spinning magnetic disk, it stores data as trapped electrical charges in silicon chips. With no moving parts, it's much more robust than a hard disk. (It's a strange term in a way as it's no more solid than a hard disk.) See All about SSDs in the Wiki.
Source Code	Source code is software in a human-readable form as created by a developer. Generally, this cannot be directly understood or run by a computer, but has to be translated into object code by a special program called a compiler, or read, interpreted and executed line by line by a program known as an interpreter.
Spam	Spam is brand of tinned processed pork. With reference to a famous Monty Python sketch, the term has been adopted to refer to any unwanted email. Such emails may simply be an irritation, such as unwanted advertising, but often it will be a scam or contain malicious attachments or links.
Spear Phishing	Spear phishing is a type of phishing attack directed at a specific victim who has probably been researched in order to make the attack seem highly plausible.
SPDT	A SPDT (Single Pole Double Throw) switch has one contact (or pole) which can be connected to one of two other contacts, so completing one or other of two alternate circuits. It's often simply called a two-way switch.
SPST	A SPST (Single Pole Single Throw) switch has one contact (or pole) which can be connected (or not) to one other contact to complete a circuit. In short, it's a simple on/off switch.
Spudger	A spudger is a tool with a thin plastic or metal blade typically used (for example) for inserting into the crack between two halves of a plastic case in order to pry it open.
Spyware	Spyware is a type of malware which reports browsing history and other personal information back to its creator without your permission.
SSD	A Solid State Disk (or SSD) is a faster (though more expensive) alternative to a hard disk for a computer's long term storage. Instead of a spinning magnetic disk, it stores data as trapped electrical charges in silicon chips. With no moving parts, it's much more robust than a hard disk. (It's a strange term in a way as it's no more solid than a hard disk.) See All about SSDs in the Wiki.
Superhet	A superhet is a common design of radio receiver in which the incoming signal, whatever its frequency, is converted into a fixed intermediate frequency to make it easier to tune and amplify. See Superhet in the Wiki.
Stepper Motor	A stepper motor is a type of motor which, instead of turning continuously, turns by a certain fraction of a revolution and stops every time it is given an electrical pulse. Used, for example, for stepping the paper through a printer as it is being printed. See DC and Universal (AC/DC) Motors in the Wiki.
Surface_Mount	Surface mount is a method of manufacture in which electronic components are soldered directly onto the surface of a printed circuit board, as opposed to the older through-hole method.
SWG	SWG or Standard Wire Gauge is a set of wire diameters where each gauge is approximately 10% thinner than the previous. For example 16 gauge is 1/16in in diameter and 50 gauge is a thousanth of an inch. Cross sectional area in square millimetres is becoming a more usual measure. See Standard Wire Gauge in Wikipedia.
Switch-mode Regulator	A switch-mode regulator is a circuit which produces a constant well defined voltage from a variable or poorly defined voltage input, by switching the power on and off very rapidly and smoothing the result. See Switching Mode Power Supplies in the Wiki.
Synchronous motor	A synchronous motor is a type of electric motor which only runs on AC, and which runs at a speed which is locked to the AC supply frequency. See Synchronous Motors in the Wiki.
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TB	A TB or Terabyte is 1000 gigabytes, but often used for 1024 gigabytes, which is more properly written TiB.
THD	THD or Total Harmonic Distortion is a measure of the degree to which an audio system distorts its input, effectively introducing new frequencies into the output.
Thermal Cut-out	A thermal fuse or thermal cut-out is a safety device which interrupts a circuit if the temperature rises above a certain critical level. Some are single use and must be replaced, whilst others reset automatically or can be reset manually. See Protection Components in the Wiki.
Thermal Fuse	A thermal fuse or thermal cut-out is a safety device which interrupts a circuit if the temperature rises above a certain critical level. Some are single use and must be replaced, whilst others reset automatically or can be reset manually. See Protection Components in the Wiki.
Thermal Paste	Thermal paste is a paste with a special formulation making it a good conductor of heat, used to improve the transfer of heat from an electronic component to a heat sink.
Thermionic Valve	A thermionic valve or vacuum tube is an amplifying device consisting of a glass tube containing several electrodes in a vacuum. An electrically heated cathode emits electrons which are attracted by an anode. Their flow is controlled by the voltage on a wire mesh grid in their path. Long since superseded by much smaller and less power-hungry transistors in all but a few niche applications.
Thermistor	A thermistor is an electronic component like a resistor but with a resistance which decreases substantially with increasing temperature. Often used for measuring or monitoring temperature, for example to warn of a dangerous temperature rise in a lithium battery.
Three Phase	Three phase is a system of power transmission in which three wires carry AC phased like three blades of a propeller. Consequently they balance out and there is no need for any wires to carry the return current. As a result, three wires can carry three times as much power as two wires in a conventional single phase circuit.
Through-hole	Through-hole is a method of manufacture in which electronic components are attached to a circuit board by passing their leads or pins through holes in the board and soldering them to the board on the other side, as opposed to the more modern surface mount technique which allows much higher component densities.
Thyristor	A thyristor, SCR or Silicon Controlled Rectifier is an electronic component with 3 leads used as a switch. See Transistors in the Wiki.
TiB	A TiB or terabyte is 1,024GiB, or 1,099,511,627,776 bytes. It is as much as can be addressed with 40 bits.
Torque	Torque is a twisting force, such as applied by a screwdriver to a screw, a spanner to a nut, or produced by the spindle of an electric motor. It is measured in Newton Metres (Nm), being the twisting force created by a linear force of 1 Newton (102 grams weight on the earth) applied to a lever of length 1m, or twice the force applied to a lever of half the length etc.
Transducer	A transducer is any device which transforms energy or a measurement from one form to another, e.g. sound to/from electricity (speakers and microphones), or temperature, light, force etc. to an electrical signal.
Transformer	A transformer consists of two (or more) coils of wire wound around a magnetic core, used to step a voltage up or down or provide isolation between two circuits. See Transformers in the Wiki.
Transistor	A transistor is an electronic component with 3 leads used to amplify a signal or switch a current on or off. See Transistors in the Wiki.
Triac	A triac is a type of thyristor which, unlike the standard type, can be used on AC. Commonly used in dimmer switches.
TRIM	TRIM is a means by which an operating system can tell an SSD about files that have been deleted. This is important for performance reasons related to the way an SSD works. See SSD Migration and Troubleshooting in the wiki.
Triode	A triode is a device with three electrodes. Most commonly used to denote a type of valve.
Trojan	A trojan is a type of malware which hides inside legitimate software. Named after the Trojan horse used by the ancient Greeks to insinuate select warriors into the city of Troy and so defeat it.
Torx	Torx is a type of security screw or screwdriver with a 6-pointed star shape. See Types of screw head and screwdriver in the Wiki.
Two Factor Authentication	Two Factor Authentication (2FA) and Multi Factor Authentication (MFA) are authentication schemes requiring a user to pass two or more tests, different in kind, in order to prove their identity. These are usually chosen from something you know (for example a password), something you have (a physical token) and something you are (a biometric such as a fingerprint or facial scan). Such a sytem is much harder to compromise than a simple password login.
Tuned Circuit	A tuned circuit comprises a capacitor and an inductor connected together. They resonate at a particular frequency as energy is continually and repeatedly passed from one to the other and back again.
Two Step Authentication	Two-step authentication is a kind of "poor man's Two factor Authentication" where the second factor is a one time code delivered to you by text message. Whilst considerably better than a password on its own, this is no longer greatly recommended on account of the relative ease by which a determined criminal could get a SIM card registered to your phone number and so get hold of the one time code. These "SIM swap" scams are endemic in some countries.
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UEFI	UEFI or Unified Extensible Firmware Interface is a program built in to newer PCs (since around 2011) which controls the PC prior to booting and initiates the boot process. (Older computers used BIOS). See BIOS vs UEFI in the Wiki.
Universal Motor	An AC/DC or Universal motor is a type of electric motor that runs equally well on AC or DC. See DC and Universal (AC/DC) Motors in the Wiki.
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Vacuum Tube	A thermionic valve or vacuum tube is an amplifying device consisting of a glass tube containing several electrodes in a vacuum. An electrically heated cathode emits electrons which are attracted by an anode. Their flow is controlled by the voltage on a wire mesh grid in their path. Long since superseded by much smaller and less power-hungry transistors in all but a few niche applications.
Valve	A thermionic valve or vacuum tube is an amplifying device consisting of a glass tube containing several electrodes in a vacuum. An electrically heated cathode emits electrons which are attracted by an anode. Their flow is controlled by the voltage on a wire mesh grid in their path. Long since superseded by much smaller and less power-hungry transistors in all but a few niche applications.
Varicap diode	A varicap diode is a type of diode which exploits the capacitance of a reverse-biased diode. This capacitance varies with the reverse voltage applied, making it useful (for example) for automatically adjusting the tuning to lock an FM radio onto a station.
Varistor	A Voltage Dependent Resistor or varistor is a device which has a very high resistance until a certain voltage is reached, whereupon its resistance falls sharply. Used for absorbing transient voltage surges in a mains supply.
VDR	A Voltage Dependent Resistor or varistor is a device which has a very high resistance until a certain voltage is reached, whereupon its resistance falls sharply. Used for absorbing transient voltage surges in a mains supply.
Virus	A virus is a term commonly used for any type of malware, but strictly meaning malware which self-replicates like a biological virus.
Volt	The Volt is the unit of electrical pressure, a bit like water pressure. Named after Italian physicist and chemist Alessandro Volta who is credited with the invention of the battery. See Electric circuits, volts amps watts and ohms in the Wiki.
Vulnerability	A vulnerability is a flaw in a website or software product by which it can be compromised for malicious purposes, whatever they might be.
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WAN	A WAN or Wide Area Network is a computer network spanning a substantial geographical area.
Watt	The Watt is the unit of power, calculated by multiplying the Volts by the Amps. Named after Scottish inventor, mechanical engineer and chemist James Watt, who invented the first truly practical steam engine. See Electric circuits, volts amps watts and ohms in the Wiki.
Wavelength	Wavelength is the distance between successive crests of a radio wave as it travels through the air. If you divide the speed of light (300,000,000 m/s) by the wavelength you get the frequency, or by the frequency to get the wavelength. See Frequency vs wavelength in the Wiki.
WiFi	WiFi is a family of wireless local area networking protocols, allowing a number of computers and other devices within a reasonably confined space to talk to each other.
Worm	A worm is a type of malware able to spread from one computer to another across a network, or across the whole Internet.
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XP	Windows XP is an obsolete Windows operating system first released in 2001. Extended support ended in 2014 and remaining users are strongly advised to migrate to a supported operating system as security patches are no longer provided.
x86	x86 is the Intel CPU architecture derived originally from the 8086 chip, released in 1978. Subsequent generations of chip were the 80286 (often referred to simply as 286), 386, 486 and 586 also known as Pentium. From 386 on, these were all 32 bit architectures, but since the mid to late 2000's the x86 architecture has been largely superseded by AMD64, though AMD64 processors retain the capability to run code written for x86.
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Yagi	Yagi is a type of aerial commonly used for terrestrial television reception, and consisting of an array of parallel rods, the array pointing in the direction of the transmitter.
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Zener diode	A zener diode is a type of diode which starts conducting in the reverse (normally non-conducting) direction at a well defined voltage, often used in electronic circuits to define a threshold or specific voltage.
Zero Day	A zero-day is a vulnerability that is found and already being exploited by malicious actors before any fix is available, giving potential victims no time at all (zero days) in which to protect themselves. Also known as an 0-Day (pronounced "oh-day").