Glossary of Terms
 
  sustain  
  1) The third segment of an envelope. It controls the level at which the envelope will hold during key-on (as long as a key is held).
2) The sustain pedal function. The level at which the amplitude envelope will hold as long as the pedal is held down. In MIDI, this is Control Change 64.
 
  Source: Yamaha  

Related terms:
  EG   MIDI   amplitude   attack   envelope