That’s Attack, Decay, Sustain, Release. The ADSR envelope is just one type of envelope, but it’s a popular one that’s been used in electronic music for decades.
Attack is the period of time after the initial release, it’s typically the loudest part of the sound
Decay is the phase while you’re going from the attack to the sustain – you’re “decaying” the volume from this sharp initial phase to the normal volume phase
Sustain is the normal phase of the sound. It is typically less volume than the attack, and go on for an indefinite period of time, or for a specific amount of time
Release is the draw down from the sustain period to no sound. A fade out.
Davíð Þór Jónsson
Written by Högni Egilsson, Stephan Stephensen and Davíð Þór Jónsson
Produced by President Bongo