feedback

F/A/V

1. unwanted noise caused by the loop of an audio system's output back to its input. 2. in a control system, data supplied to give an indication of status, i.e., on or off.

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TV

[1:video] Infinite loop of visual patterns from signal output being fed back as input; achieved by aiming live camera at receiving monitor. [2:audio] Echo effect at low levels, howl or piercing squeal at extremes, from audio signal being fed back to itself.

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WikiB

A loud noise produced when the amplified sound from an output ( loudspeaker ) is picked up by an input ( microphone, phonograph ) feeding that loudspeaker. This can be potentially damaging to both the speaker(s) in question, as well as the hearing of the subjected listener. This may also occur when an input is directly patched into an output of the same device, usually due to operator error.

In radio broadcasting, feedback may occur when a DJ increases his or her headphone volume to a high enough level that the microphone is able to pick up the sound coming from the headphones, usually when the DJ's head is turned to one side.