Modifier

any signal that changes the audio signal in some way.

source: http://www.soundonsound.com/sos/jul99/articles/synthsecrets.htm

or:

Modifiers are those parts of the synthesizer that modify the signals produced by other parts of the synth. Most obviously, these are the filters and amplifiers..

source: http://www.soundonsound.com/sos/jul04/articles/synthsecrets.htm