Phase modulation (PM) is a synthesis technique in which the phase of a carrier signal (usually an oscillator) is modulated by the amplitude of another signal called the modulator. The modulator signal can be another oscillator, an LFO, or any other source of control voltage (CV).
The concept of phase modulation is closely related to frequency modulation (FM). In fact, phase modulation can produce similar results to frequency modulation, as they are mathematically equivalent under certain conditions. However, the way they achieve those results is different.
In phase modulation, the modulator signal directly affects the phase of the carrier signal, whereas, in frequency modulation, the modulator affects the frequency of the carrier. This results in slightly different behavior and sonic characteristics between the two methods, especially when modulating signals with complex waveforms or multiple modulators.