Envelope generator with Attack, Decay, Sustain, and Release parameters. This is a simple type of envelope generator and was first used on early analog synthesizers. This form of envelope generator continues to be popular on modern instruments. See Decay for more details.

Some content below is with permission of Wikipedia.

ADSR

When a mechanical musical instrument produces sound the loudness of the sound changes over time in a way that varies from instrument to instrument. The "attack" and "decay" of a sound has a great effect (along with its spectral content) on the instrument's sonic character. Sound synthesis techniques often employ an envelope generator that controls a sound's parameters at any point in its duration. Most often this is an "ADSR" (Attack Decay Sustain Release) envelope, which may be applied to overall amplitude control, filter frequency etc. The envelope may be a discrete circuit or module or implemented in software. The contour of an ADSR envelope is specified using four parameters:

* Attack time is the time taken for initial run-up of level from nil to peak.
* Decay time is the time taken for the subsequent run down from the attack level to the designated sustain level.
* Sustain level is the amplitude of the sound during the main sequence of its duration.
* Release time is the time taken for the sound to decay from the sustain level to zero after the key is released.

This article is a stub; some addition of images and further discussion would be very useful.