ADSR
The four-stage model of how a note starts, falls, holds and dies away — a synthesizer's way of admitting that real instruments are not simply switched on and off.
Attack is the time to reach full level once a key goes down, decay the time to fall to the sustain level, and release the time to silence after the key comes up. Sustain is a level, not a time — the single most common misunderstanding on beginner forums. The model grew up around Robert Moog's envelope generators in the mid-1960s, at the suggestion of composer Vladimir Ussachevsky.
An envelope can drive more than volume: route it to the filter and you get the classic "yow" sweep of a synth bass. Home-computer sound chips put ADSR in silicon — the C64's SID had three, with quirks that needed a hard restart before they behaved.