VCO
"That's it! Voltage control!"
— Korg MS-20 owner's manual
A VCO, short for voltage controlled oscillator, is exactly what it sounds like — an oscillator controlled by a control voltage. The higher the voltage, the higher the oscillator's frequency (pitch).
This control voltage could come from pretty much anything, such as a keyboard, envelope generator, LFO, the output of another oscillator, or a patch point that lets you use anything to control it.
See also
Synthesis: Envelope generator | Footing | Frequency shifter | Noise | Oscillator | Periodic waveform | Program (synthesiser) | Pulsewidth modulation | Sub-oscillator | VCA | VCF | VCO
