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:

L(f) = 10log[0.5(S φ(f ))]

where S φ(f ) is the spectral density of phase fluctuations.

Source: Federal Standard 1037C