There are four stages of an Attack, Sustain, Decay, Release (ADSR) envelope. Attack, decay and release are temporal parameters while sustain is a level parameter.
ASDR Envelope
(courtesy ASDR Envelope Explained)
(click to enlarge)
Attack
The attack stage of the ADSR envelope is the initial stage that dictates the time it takes for that particular sound to rise in volume from its minimum value to its maximum value.
Decay
Decay is the next stage of the envelope. It comes into effect as soon as the attack reaches its peak. Once that happens, the decay will bring the volume down to the level of sustain.
Sustain
Unlike the previous two stages, sustain is not a temporal parameter. What this means is that once you reach sustain stage, the volume of your note will remain the same.
Release
The note starts to fade out. Similar to attack and decay, the speed with which it will fade out depends on the length of the release stage. [https://www.musicskanner.com/attack-sustain-decay-release-envelope-explained/|ASDR Envelope Explained]
See Also