Envelope, as it relates to a sound wave, can be broken up into four main parts over time.

The first is the sound's attack, or the initial peak of the sound wave. The second is the sound's sustain, or the plateau of the sound wave after the initial peak, but before losing energy. The third is the sound's decay, starting at the point the wave begins to decrease in amplitude until the forth stage, release, the part of the sound that trails off into the noise floor.