Hz/V, or hertz per volt, is a pitch control standard in which an oscillator’s frequency rises in direct proportion to the control voltage. Doubling the voltage doubles the frequency.
Because an octave is a doubling, the voltage needed for each octave grows as you go up, which makes Hz/V less convenient than 1V/oct for scales and quantizers. Some Korg and Yamaha instruments use it, and a converter is needed to link them to 1V/oct gear.