ADSR envelope
Also said as Envelope, Attack, decay, sustain, release, Volume envelope
Four numbers that shape a note's loudness over time. Attack is how fast it rises, decay how fast it falls to the sustain level, and release how long it takes to fade once the note ends.
The same oscillator can be a pluck, an organ or a slow pad, depending only on these numbers. A short attack also feels instant, while a slow one on a button sound feels like lag.
Say it
“Give the UI click a 2 ms attack and an 80 ms decay with no sustain, so it feels instant. The menu pad can take 700 ms to fade in.”
Where you’ve seen it
The Commodore 64's SID chip gives each of its three voices its own attack, decay, sustain and release, in hardware.
Not to be confused with reverb
An envelope shapes the sound itself. Reverb adds the room's echoes after it, so a short note in a large hall still rings on.