Envelope Generator

"A circuit capable of generating a control signal which represents the envelope of the sound you want to recreate. This may then be used to control the level of an oscillator or other sound source, though envelopes may also be used to control filter or modulation settings. The most common example is the ADSR generator."

source: http://www.soundonsound.com/information/Glossary.php#E

Here's something extra from the same site:

"A good definition of an envelope is this: the graph of the way a parameter changes over time is a visual representation of its envelope. But, in a typical synthesizer, you can — at any given moment — change the value of a parameter using any number of modifiers. The total envelope, therefore, may be the sum of the simultaneous actions of numerous devices. So here's this month's first Synth Secret (and it's a biggie!):

What we usually call an envelope generator may be only one contributor to the true envelope of a given parameter.

It is for this reason, perhaps, that some of the earliest synthesizer manufacturers adopted a different term for the devices that we now call Envelope Generators. They called them 'Transient Generators'. It's a more precise term…"

source: http://www.soundonsound.com/sos/nov99/articles/synthsecrets.htm

Controls the parameters of sound at any given point over time. An ADSR Envelope Generator is the most common.

Attack time is the time taken for initial run-up of level from nil to peak, beginning when the key is first pressed.
Decay time is the time taken for the subsequent run down from the attack level to the designated sustain level.
Sustain level is the level during the main sequence of the sound's duration, until the key is released.
Release time is the time taken for the level to decay from the sustain level to zero after the key is released.
Chris Ozone