Chromagram
A spectrogram folded down to twelve rows, one per note of the chromatic scale, regardless of octave.
All the energy belonging to a C — whether at 65, 131 or 523 Hz — goes into one bin, and likewise for C#, D and so on up to B. What remains is a twelve-dimensional vector per frame showing which pitch classes are sounding, but not in which register or on which instrument. Takuya Fujishima used the idea in 1999, calling it a pitch class profile, to recognise chords, and that remains its main job.
Losing the timbre is the point. The same song played on acoustic guitar and by a symphony orchestra produces similar chromagrams, so the technique is used for cover-song detection, key estimation and aligning recordings with a score. The Western twelve-tone division is baked in, of course, which makes it a poorer fit for music that slices the octave differently.