CV/gate (control voltage)
How analogue synths were controlled before MIDI: a voltage sets pitch or another parameter, a gate says when the note sounds.
CV is a continuous voltage that drives something — usually pitch, but just as easily filter cutoff or level. Moog and most others used 1 volt per octave, so one more volt always means one octave up. Korg and Yamaha went with Hz per volt, and the two schemes can't talk without a converter. The gate is a separate signal held high while a key is down, typically firing an ADSR envelope.
MIDI displaced CV/gate in the early 1980s because one cable could carry many channels. But a voltage has no seven-bit resolution and no serial delay, and thanks to Eurorack it is back as the lingua franca of modular synths.