A voltage controlled filter circuit that uses operational transconductance amplifiers, or OTAs, as processing and control elements. The circuit relies on a property of the OTA, which is that as its bias current changes, its transconductance also changes, which (somewhat of an over-simplification) allows the OTA to be used as, in effect, a voltage controlled resistor. This is used as the resistance in a basic RC filter circuit (in this case properly called a gm/C filter), and so by using a control voltage to vary the transconductance of the OTA, the R-C time constant and hence the cutoff frequency can be varied.
The OTA filter has somewhat different sonic characteristics than the more common transistor ladder VCF design. It is often designed as a "smoother" filter, with less internal distortion (assuming the circuit is designed properly). It has the advantage of being inherently resistant to control voltage feedthrough. OTA circuits are more of a challenge to design such that they respond to control voltage in a linear fashion, and do not suffer from excessive noise.