Angle modulation in which the phase angle of a carrier, such as an electronic, radio, or optical carrier, is caused to depart from its reference value by an amount proportional to the instantaneous value of the modulating signal. Common abbreviation PM. See also  angle modulation,  carrier,  continuous phase modulation,  deviation ratio,  differential phase-shift keying,  electronic,  frequency modulation,  instant,  modulation index,  optical carrier,  phase,  phase deviation,  phase difference,  phase-frequency distortion,  radio,  reference,  signal,  value.