The part of an amino acid that confers its identity. Side chains range from a single hydrogen atom (for glycine) to a group of 15 or more atoms.

A side chain in sound dynamics processing is a feature of compressors that allows for the processor to be triggered by an external signal as opposed to the signal that it processes. Spectral processing, compression keying, and ducking applications are all based on this principle, and it is also a feature of noise gates that is useful for synching external sounds to recorded tracks.