VCO (voltage-controlled oscillator)
The analogue synth's tone source, whose pitch is set by an incoming voltage.
A VCO puts out a periodic waveform — sawtooth, square, triangle, sometimes sine — and lets a control voltage set its frequency. Moog's one-volt-per-octave convention became the de facto standard that modular systems and Eurorack still follow. Lock two oscillators together with hard sync and you get the tearing sound half the leads of the eighties were built on.
Stability was always the catch. Analogue oscillators drift with temperature, and a synth needed time to warm up before it would stay in tune. Roland's answer in 1982 was the DCO in the Juno-6: a digitally clocked oscillator with analogue wave shaping. Purists complained that it sounded too stable, which tells you something about purists.