An optional control on a voltage controlled oscillator (or the digital equivalent of one) that shifts the oscillator's response by one or more exact octaves. This is often used, for instance, in a voice architecture having more than one oscillator to have one play an octave lower than the other, for reinforcement of bass tones, for instance.
An octave shift control on a modular VCO usually takes the form of a rotary switch, which may allow four or five choices. Often these are numbered in "footage", as in the notation used to indicate ranges on pipe organs, e.g., if 8' indicates a "middle" octave, 4' would be one octave above, 16' one octave below, and so on. The control might also have a setting that shifts the VCO into a low frequency range, allowing the VCO to be used as an LFO.