(1) The portion of an envelope that comes after the sustain portion and is the amount of time it takes for the sound to go from the sustain level back to silence. The release segment begins after the key is lifted, i.e., the release time to zero-level after a MIDI Note-Off message arrives or the key is released. See ADSR.
(2) The time it takes for a compressor’s gain to come back up to normal once the input signal has fallen back below the threshold.