(1) Amplification. The increase in a signal that is transmitted from one point to another through an amplifier. A material that exhibits gain rather than absorption, at certain frequencies for a signal passing through it, is known as an active medium. (2) A general term used to denote an increase in signal power in Transmission from one point to another. Gain is usually expressed in decibels.(3) In a photo detector, the ratio of electron-hole pairs generated per incident photon.