A circuit that combines (sums) audio signals or control voltages. Most people are familiar with the console-type audio mixers seen in recording studios; these are usually complex units that contain additional processing. However, synths often include their own basic mixer circuits, owing to the fact that many synths have multiple sources of audio signals internally. A mixer that is intended to sum control voltages may be referred to as a "summer" or "adder"; such a mixer must be DC coupled. Control voltage mixers are often embedded in other circuits or modules; for example, a VCO with both a control voltage input from the keyboard, and a separate FM control voltage input, usually combine the two control voltages with an internal mixer.
It is worth noting that radio engineers use the word "mixer" to refer to a class of modulating and frequency changing circuits. These are not at all like an audio mixer. In fact, one such circuit is the one that synth players refer to as a ring modulator.