Glossary:Phase
Two sounds (or electrical signals, or radio or light waves) may be identical, but if the waves of one lag in time behind those of the other, they're said to differ in phase. A phase difference is measured in degrees; 360 degrees represents a complete cycle so is the same as no phase difference, whereas with a phase difference of 180 degrees, the peak of one wave will coincide with the trough of the other and combined, they will cancel out.