Relating to the waveform characteristics of an AC signal such as an audio waveform. It decribes the ratio between the peak amplitude and the RMS level. The 'peakier' or more dynamic the signal, the greater the crest factor. Crest Factor is normally expressed in decibels.
A pure sine wave has a crest factor of 1.414 or 3.01dB, while a triangle wave is 4.77dB and a square wave 0dB.