noun: modulation of the frequency of a (radio) carrier wave or acoustic (sound) wave. Modulation can be additive or subtractive.

A periodic change in the pitch of a sound; for instance, vibrato. [Learning Music with Synthesizers]

"More commonly, FM, is a method of radio broadcasting in which information is transmitted by varying the frequency of the broadcast signal. It is better suited to high- fidelity transmission of music than ordinary radio, which operates on a principle known as amplitude modulation, or AM. Broadcast standards in the U.S. are such that FM broadcasts have a wider frequency range (up to 15,000 Hz, as compared to about 5,000 Hz in ordinary AM radio), freedom from static and noise, and a greater spread between loud and soft passages." [The True Sound of Music - A Practical Guide to Sound Equipment for the Home]

See Also

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Amplitude Modulation
Frequency
Frequency Response
Modulation
synthesis
synthesizer
1.20 - Evolution and Devolution of Frequency
15.20 - Dissociation Frequency