The final part of a sound where the amplitude diminishes due to a
loss of energy.
The intensity of a sound is generally not a static quantity, but
changes over the duration of the sound. This change in the intensity
of a sound over time is called an ENVELOPE. Thus, at the beginning
of a sound, the intensity does not change in an instant from silence
to peak level; rather, it requires a certain amount of time, called
the ATTACK TIME (or simply, the ATTACK), to do so. Similarly,
at the end of a sound, the intensity does not suddenly plummet to
silence. The amount of time required for the intensity of the sound
to diminish to nothing is called the DECAY TIME (or simply,
the DECAY), or the RELEASE.
from Barry Truax's Handbook for Acoustic Ecology.