In the world of analog synthesizers, an expander is a unit that is intended to add capabilities to an existing synth. Commonly, an expander will contain functions and features such as additional VCOs, VCFs, and LFOs, auxiliary circuits such as mixers, or functions not found in many analog synths (e.g., a ring modulator). Expanders generally have no self-contained keyboard or other means of playing them directly; they interface to other synths via CV/Gate and analog audio inputs and outputs. Many expanders are semi-modular so that the user can patch in and use only the functions desired at a given moment.
Probably the best known example is the Oberheim Synthesizer Expander Module, which became a synth in its own right when Oberheim used it to develop the Four Voice, one of the first polyphonic synths. Other notable expanders have included the ARP Little Brother, the Korg MS-50, and the EML Electrocomp 200.