1. The instantaneous variation of the frequency of a carrier wave in response to changes in the amplitude of a modulating signal, which provides a more static-free transmission of radio waves than AM (amplitude modulation) broadcasting.
2. Variation of the frequency of a carrier wave (commonly a radio wave) in accordance with variations in the audio signal being sent.

Developed by American electrical engineer Edwin H. Armstrong in the early 1930's, FM is less susceptible to outside interference and noise; such as, thunderstorms, nearby machinery, etc. than is AM.

Such noise generally affects the amplitude of a radio wave but not its frequency, so an FM signal remains virtually unchanged.

FM is also better able to transmit sounds in stereo than AM and commercial FM broadcasting stations transmit their signals in the frequency range of 88 megahertz (MHz) to 108 MHz.