Maybe it would be helpful to point out that crest factor is the ratio of peak to RMS value (not peak-to-average as started once in the referenced web page).
The crest factor varies with the waveform. Thus a sinusoid has a crest factor of 1/√2, a triangle or sawtooth wave has a crest factor of 1/√3, and a square-wave has a crest factor of 1. It is typically used when specifying true RMS meters, as there is a limit to the value of crest factor that they can accurately measure. Generally the higher the crest factor they can accommodate, the better the meter.