Decay describes the progressive reduction in the amplitude or volume of a sound or electrical signal over time. In the context of acoustics and reverberation, it refers to the time it takes for sound to lose energy and decrease in volume until it becomes inaudible after the sound source has stopped emitting sound.

In the context of synthesizers and ADSR-type envelope generators, the Decay phase begins immediately after the Attack phase has reached its maximum level. During this stage, the volume or controlled parameter descends from the initial peak to the level defined for the Sustain phase. This determines how quickly the initial impact of the sound transitions to its resting or sustaining state while the note remains pressed.