Crest factor
The ratio between a signal's highest peak and its average level — one number that gives away how hard a sound has been squashed.
Peak divided by RMS, usually quoted in decibels. A pure sine sits at 3 dB, a square wave at 0 dB. Acoustic music with untouched dynamics easily reaches 15–20 dB, because drum hits and plucked notes stick far above the average. The lower the figure, the more work the compressors and limiters have done.
During the loudness war crest factor was the clearest measure of the damage: peaks pinned to the ceiling, average level shoved up to meet them. The number matters outside music too — amplifiers and radio transmitters are sized by crest factor, because the peaks are what clip and the average is what generates heat.