A malfunction occurring in some amplifier circuits, in which the device will feed back upon itself internally. The possible causes of this are numerous and can be complex (beyond the scope of a Word For The Day). In audio equipment, this may generate an ultrasonic signal that will occur only when an input signal is present and generally only during part of the audio waveform. Although the ultrasonic component isn’t directly audible, its existence modulates the signal causing audible distortion, and is potentially damaging to tweeters. Tube amp circuits tend to be prone to more different variations of this phenomenon than most solid-state designs. Most modern audio amp designs incorporate circuitry to prevent this problem to the extent that it’s generally not much of a concern these days.