Voltage Controlled Oscillator (VCO)   

An oscillator in an analogue synthesizer which changes its frequency according to changes in a control voltage.   

Standard is usually 1 volt = change of one octave.

VCOs may have:
outputs: various waveforms: sine, pulse, triangle, sawtooth
output controls: for coarse and fine frequency, wave shape (such as pulse width) and output level as well as
inputs: synchronization to another oscillator, pulse width (for pulse width modulation) and frequency modulation
input controls: for control voltage attenuation, synchronization to another oscillator, and frequency modulation.