The 'shape' of a sound in relation to time and volume.
Attack, Decay, Sustain and Release are the four parameters found on a basic synthesizer envelope generator, and they match the physical attributes of naturally occurring sound.
The Attack, Decay, and Release parameters are rate or time controls. Sustain is a level. When a key is pressed, the envelope generator will begin to rise to its full level at the rate set by the attack parameter, upon reaching peak level it will begin to fall at the rate set by the decay parameter to the level set by the sustain control. The envelope will remain at the sustain level as long as the key is held down. When a key is released, it will return to zero at the rate set by the release parameter.