Adaptive music
Also said as Interactive music, Dynamic music, Reactive score
A score that responds to what the player is doing, by bringing layers in and out, moving between sections, or both. The composer writes rules as well as notes.
A film composer knows what happens at each second, and a game composer does not. Adaptive music lets the score stay matched to a scene that plays differently every time.
Say it
“Drive the music from one danger value. Above 20%, fade in bass, above 45% drums, above 65% lead. Above 75%, move to the alert section at the next bar line.”
Where you’ve seen it
LucasArts' iMUSE, first used in Monkey Island 2, has been likened to a pit orchestra: it plays shorter or longer sections while it waits for key events. Red Dead Redemption instead layers stems in one key and tempo, so they can be combined during play.
Not to be confused with dynamic mix
A mix changes how loud sounds are. Adaptive music changes what the music itself plays, and when.