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.