The information signal is used to vary the carrier signal frequency. The phase of the carrier varies with the integral of the modulating wave.
Signal-to-noise ratio may be increased over amplitude modulation without increasing power (though bandwidth will suffer).
Ratio of the maximum frequency deviation to the maximum modulating frequency of a frequency modulation system, under specified conditions.
In frequency modulation with a sinusoidal modulating wave, the ratio of the frequency deviation of the modulated wave to the frequency of the modulating function.