Sustain

The third of the four stages of a sound (Attack, Decay, Sustain and Release, or the ADSR envelope), the sustain is the part of the sound that holds at approximately the same volume after the initial attack and drop in volume level (decay), until the sound stops playing. In some instruments, sustain is short or virtually nonexistent; in many instruments, sustain loses volume over time until the sound dies off (for example, in a held piano note). In synthesizers and samplers, the sustain function can be set to hold the note at the same volume level indefinitely until the key is released.