Envelope

From AudioLexic

The envelope is one of the essential tools of sound synthesis. The abbreviation for it is ENV. Usually it covers the ADSR. "A" for Attack, "D" for Decay, "S" for Sustain and "R" for Release. This is the common case, but on some synthesizers the envelope can have more or less stages. A complex envelope will have more than 4 parameters, 5, 6, even more… A simple one, AR or ASR will have either only these 2 parameters or fixed levels for the missing ADSR elements.

An envelope can be reversed too, that is the action to activate the envelope will have the reverse effect of the normal envelope. for example, cut the sound instead of letting it play when you play a note.