Audio feedback (also known as Larson effect) is a special kind of feedback which occurs when a loop exists between an audio input (eg. a microphones) and an audio output (eg. a loudspeaker). For example, a signal received by the microphone is amplified and passed out of the loudspeaker, the sound from the loudspeaker is received by the microphone again, amplified further and then passed out through the loudspeaker.
The frequency of the resulting sound is determined by resonant frequencies in the amplifier and transducers, the acoustics of the room, the directional pick-up pattern of the transducer, and the distance between input and output.
Audio feedback is usually not wanted for microphones and acoustic instuments. However, electric guitar players such as Jimi Hendrix have been using it as an audio effect, supposedly invented by the Monks.