Glossary:3-Terminal Regulator
A 3-terminal regulator is a device which takes an unregulated input voltage and produces a constant well-defined output voltage. It has just 3 connections (or terminals): the input, the output and a common ground. The 78/79 series integrated circuits are a very common example. 78 devices take positive input and output whereas 79 series take negative. Two further digits in the type code indicate the voltage, for example 7805 produces +5V output.