Phase noise
In an oscillator, phase noise is rapid, short-term, random fluctuations in the phase of a wave, caused by time domain instabilities.
Phase noise, L(f) in decibels relative to carrier power[?] (dBc[?]) on a 1-Hz bandwidth, is given by:
where S φ(f ) is the spectral density of phase fluctuations.
Source: Federal Standard 1037C