An Etymological Dictionary of Astronomy and Astrophysics

General: The modification of some property of a phenomenon by another distinct phenomenon.
Physics: Variation of some characteristic (amplitude, phase, or frequency) of a radio wave,
called the carrier wave, in accordance with instantaneous values of another wave, called the modulating wave. → amplitude modulation; → frequency modulation.
Optics: A synonym for contrast, particularly when applied to a series of parallel lines and spaces imaged by a lens.

See also: Verbal noun of → modulate.