Specifically, a type of music called Black MIDI. Black MIDI is named as such because the idea is to include as many notes as possible into a short little arrangement. Making a score of such an arrangement would be such a befuddlement of notes and accidentals and other such things that the staff would be just a block of black ink—hence, Black MIDI.

Also, it would be physically impossible to play “by hand.”

Example of such a thing:

(Note: this may glitch out near the middle, depending on the speed of your compy)

It’s actually kind of cool-sounding, huh?