ADSR is an acronym that represents the four parts of a sound's envelope
Attack is the time taken for initial run-up of level from nil to peak, beginning when the key is pressed.
Decay is the time taken for the subsequent run down from the attack level to the designated sustain level.
Sustain is the level during the main sequence of the sound's duration, until the key is released.
Release is the time taken for the level to decay from the sustain level to zero