The act of letting a musical note ring or sound out without cutting it off. In keyboard and synthesizer envelopes such as an ADSR the sustain parameter determines how loud a sound will ring out while the note is held. A sustain pedal is also used on keyboards to allow notes to continue to sound even after the keys have been let up by the player. In electronic keyboards sustain is a continuous controller message (CC # 64) that transmits the sustain information. Though technically it is a continuous controller (by virtue of being on the list) it doesn’t really work in a continuous fashion, as sustain is an on/off function. So there is a (generally) high value to signify the depression of the sustain pedal and a low value to signify it not being depressed. This is generally interpreted as sustain on or off and acted upon according to the workings of the receiving device.