VCF
"That's it! Voltage control!"
— Korg MS-20 owner's manual
A VCF, short for voltage controlled filter, is exactly what it sounds like — a filter controlled by a control voltage. The higher the voltage, the higher the filter's cutoff point. (It's very rare for a filter to sport voltage controlled resonance.)
This control voltage could come from pretty much anything, such as a mod wheel, envelope generator, LFO, or a patch point that lets you use anything to control it.
It's quite common for synthesisers (especially Roland's, such as the Jupiter-8 and Juno-106) to feature a non-voltage-controlled highpass filter and a voltage controlled lowpass filter, or for that matter a non-resonant highpass filter and a resonant lowpass filter. As lowpass filtering tends to be more common in nature, making things brighter and duller, the lowpass filter tends to be used more expressively than the highpass filter.
The most versatile filters are multimode, and provide voltage control of the cutoff point while the filter can be lowpass, highpass, bandpass or notch. The filters in Doepfer's A-100 tend to be great for this.
See also
Synthesis: Envelope generator | Footing | Frequency shifter | Noise | Oscillator | Periodic waveform | Program (synthesiser) | Pulsewidth modulation | Sub-oscillator | VCA | VCF | VCO
