A device for holding, for example, the output volume of a radio receiver or the recording level on a tape recorder substantially constant despite variations in the input signal. The term also covers the process involved. A variable gain element in the receiver is controlled by a voltage derived from the input signal. Variations in the size of the input signal cause compensatory changes in gain in the receiver. Biased automatic gain control is a process that comes into operation only for signals above a predetermined level.

In radio receivers with automatic gain control, the AGC action can result in maximum gain when no carrier is being detected. This can produce a significant amount of noise that is heard by the listener. It can also result in receivers that automatically scan for the next radio transmission. To avoid the annoying noise when no carrier is being detected, the output of the receiver is switched off or muted. See also squelch circuit.