Sidechain compression
Compression where a signal other than the one being processed decides when the compressor clamps down.
An ordinary compressor listens to its own input. Give it an external key input and it listens to something else instead — the presenter's microphone, the kick drum in a dance track — and pulls the music down whenever that signal appears. In radio and podcasting this is called ducking, and the aim is for nobody to notice.
Dance music made noticing the point. Key the mix from the kick, set a long release, and the whole track breathes in time: the pumping sound that defined French house and early-2000s EDM, now often faked with a volume curve rather than an actual compressor. The same idea, with a filter in the key path, is what makes a de-esser and a keyed noise gate work.