A characteristic of many of the types of voltage controlled filters used in synths, in which the filter can be made to produce a tone without any input signal. The resonance adjustment of many such filters is basically a positive-feedback circuit. If the resonance value is turned up sufficiently high, the self-noise of the filter circuit will cause the positive feedback to "ring" the circuit and produce a tone. The resulting waveform is usually fairly close to a sine wave.
At one time, self-oscillation was regarded as an undesirable characteristic in a VCF, and in the early days, several attempts were made to design a VCF with anti-oscillation circuitry. These had a number of problems; many worked by decreasing the filter output level as the resonance was turned up, so that at very high resonance settings the filter became nearly inaudible. The ability to self-oscillate is now regarded generally as a feature of a analog filter; among things, it allows the filter to be used as a VCO producing a sine wave on synths that otherwise lack a means of producing a sine wave. Digital filters generally do not self-oscillate, although is possible to design one that does so.