Feedback
The return of a portion of the output signal back into the input of a system. This can be done in a controlled manner through a feedback circuit to alter the sound of an instrument (most commonly electric guitars or analog synths). It can also describe the unwanted feedback loop created when an open microphone is picking up the sound from a nearby speaker, generating a loud, oscillating frequency that increases in intensity until the feedback loop is broken by turning off the mic or speaker, or by use of an equalizer to attenuate the frequency.
