Abbreviation for Voltage Controlled Filter. The VCF is to filtering what the VCA is to amplifiers. Actually many filters are amplifiers where the gain of the amp is manipulated by other components such that certain frequencies are filtered out of the final output. And this is exactly what a VCF is. The user has control over the cutoff frequency of the filter and whether it is low pass, hi pass, or band pass. More advanced designs allow you to add resonance and/or vary the Q of the filter.