In audio, sidechain refers to a signal which has branched away from the audio path of a device, and will not appear on the output. It usually controls a processor which does affect the audio path directly.

For example, a compressor or gate takes a sidechain, and determines its level, using this to control a VCA which directly affects the output.

                                   Amplifier
                  Signal             |\
Audio In -----+----------------------| >------  Output
              |                      |/
              |                       ^
              |   +--------------+    |
    Sidechain +---|level detector|----+
                  +--------------+
Some devices bring the sidechain to the panel, to allow an external signal to control the processor, or to have a processed sidechain sent to another device. Examples of such devices are DBX 1066 and BlueMAX stereo compressor, which allow a signal to be compressed according to the output of another signal.