Envelope

The way in which the level of a sound or signal varies over time.

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

or:

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.

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

or:

Factors which determine how a signal / sound changes over its period of existence.

source: http://www.zonicweb.net/music/musictechnologydictionary.htm