Ducking
Also said as Sidechain, Sidechain compression, Auto-ducking
Turning one sound down automatically while another plays, most often music under speech. A sidechain is the wire that does it: one sound's level controls another's volume.
It keeps dialogue and big moments clear without anyone hand-mixing each one. If it comes back too quickly the music pumps, which is why the release is slower than the attack.
Say it
“Sidechain the music to the dialogue bus: 9 dB down with a 50 ms attack and a 400 ms release, so it never pumps between words.”
Where you’ve seen it
Wwise has both auto-ducking and sidechaining, the latter built with a meter effect on one bus driving a game parameter on another. DICE contrasted its HDR audio with traditional ducking, which shows how standard ducking had become.
Not to be confused with mix snapshot
A snapshot is a set of levels you choose for a situation. Ducking follows a live signal and reacts to whatever is actually playing.