A signal intended to control some other parameter, such that the value of that parameter corresponds to the input voltage. For instance, a voltage controlled oscillator might have a control voltage input which determines its output frequency. In an analog synthesizer designed according to the principle of voltage control, control voltages and audio signals are interchangeable, such that, for instance, the output of a VCO could be used as a control voltage input to the cutoff frequency of a voltage controlled filter.
The principles of voltage control were developed in the early 1960s by Don Buchla and Bob Moog, working independently. Their work was in turn based on some circuits designed for the RCA Synthesizer in the 1950s, and the work of Lamonte Young in the 1940s. Voltage control was a major step forward; prior to that time, synth parameters were usually controllable only manually by knobs or switches, which severely limited the playability and expressiveness of the instrument.