ADSR

A - Attack: time taken for initial run-up of level from nil to peak, beginning when the key is first pressed.
D - Decay: time taken for the subsequent run down from the attack level to the designated sustain level.
S - Sustain: level during the main sequence of the sound's duration, until the key is released.
R - Release: time taken for the level to decay from the sustain level to zero after the key is released.

http://en.wikipedia.org/wiki/Synthesizer#ADSR_envelope