Sidechain
Definition
A signal routing technique in which the control input of a dynamics processor is driven by a different signal than the audio being processed. Allows one signal to control the dynamics of another.
In Simple Terms
Using one sound to control the effect on another. The classic example: every time the kick drum hits, the bass automatically ducks out of the way for a split second, then comes back. It creates space without you having to automate anything by hand.
In Practice
A kick drum is routed to the sidechain input of a bass compressor. Every kick hit briefly compresses the bass, creating space for both elements to coexist without masking each other — a fundamental technique in modern mixing.