The last portion of a standard ADSR envelope found in synthesizers. The release portion of the envelope generator determines how long it takes the sound to decay (or fade out) after the key has been released. Pad and string sounds often have slower release times than articulate bass or piano sounds.
Also pertains to the release time control found on noise gates and compression units. This parameter determines how long it takes the device to return to it’s normal state (not gated or compressed) after the signal has risen above (gate) or fallen below (compressor) the threshold.