Adaptive music
Game music that changes with the state of play — ramping up in combat, thinning out while exploring, dropping a stinger on a kill.
Two core techniques. Horizontal re-sequencing swaps between pre-composed segments at musical boundaries; vertical layering mutes and unmutes stems stacked on top of one another. The hard part is never the audio but the transition: changing musically on the beat instead of hard-cutting.
LucasArts' iMUSE (1991, Monkey Island 2) was the landmark system, built precisely to sync transitions to the beat so no cut sounded abrupt. The patent was authored by Michael Land and Peter McConnell. Today most of it is handled by middleware such as FMOD and Wwise, but the problem they solve is the same one Land wrestled with at the piano.