In radio, frequency deviation is the result of frequency modulation. In the FM band for broadcasting, deviation is normally 150kHz total, which is ±75kHz of the carrier frequency. Excessive deviation is the result of overmodulation, and causes interference to adjacent channels.
Phase modulation has the same net effect as FM, but phase deviation is measured in degrees or radians, compared to the umodulated carrier wave. The modulation index is equal to the phase deviation in radians.
See also: standard deviation in mathematics