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