This is a function on a filter in which a narrow band of frequencies (also known as the resonant peak) becomes relatively more pronounced. If the resonance is set high enough, the filter will begin to self-oscillate, producing a sine wave audio output even without a note being played on the keyboard. Filter resonance is also sometimes referred to as emphasis or Q. In some of the oldest synthesizers, this effect is referred to as regeneration or even feedback, for the simple reason that feedback was used in the circuit to produce the resonant peak.
