An Acronym for Attack, Decay, Sustain, Release.

This is the canonical type of sound synthesizer. All synthesizers generally contain some type of sound wave which can be edited by an envelope that controls the wave's development over a note's life. This development is usually divided into the phases Attack, Decay, Sustain, and Release. All of the four stages can be controlled and modified in their length and duration.

During the Attack phase, the sounds ramps from silence to an initial peak. This might be gentle, gradual for an emulation of a melancholic violin, or very rapid for a forcefully stricken piano.

The Decay phase controls the development of the sound right after the Attack's peak. In emulating a violin, this might not be much of a decay at all, as a violin player can hold a note for a very long time. Conversely, in a piano emulation, the note will decay almost as quickly as its attack was.

The Sustain phase is what happens on keyboard synthesizers if you keep a key pressed beyond the Decay phase. A piano emulation has no Sustain phase to speak of, while for a violin it might be quite long.

Finally, the Release phase controls the note's fading to silence.

Although an emulation of a real instrument will have all three phases after the Attack fade the sound towards silence to varying degrees, there is no technical requirement for this at least in the Decay and Sustain phases. Most synthesizers will insist on fading to silence during Release however.