Abbreviation for Voltage Controlled Oscillator. It is an oscillator whose pitch (or frequency) is controlled by an input voltage. In a keyboard, for example, pressing different keys produce different voltages, which then drive the oscillator circuit to produce specific pitches (notes). Modern (digital) keyboards don’t work this way anymore, but back in the days of analog synthesizers it was all done with voltage. A lot of the old stuff was one volt per octave. So if it took one volt to go from a low C to the C an octave higher, it took an additional volt to reach the next C up, and so on.