(1) The third of the four segments in an ADSR envelope. The sustain portion of the envelope begins when the attack and decay portions have run their course, and continues until the key is released. The sustain control is used to determine the level at which the envelope will remain until the note is released. While the attack, decay, and release controls are rate controls, the sustain control is a level control.
(2) A pedal fitted to pianos and some percussion instruments, such as the vibraphone, that is used to prevent notes from being damped, thus lengthening their decay time, and therefore, not a true (infinite) sustain. A pedal offering a similar function is often available for synthesizers where, of course, an infinite sustain can be achieved on some voices.
(3) A MIDI Controller Change message assigned to the parameter in a synthesizer which reproduces the function described in sustain (2).