Feedback

Generically, a portion of the output of a process returned to the input, often for control of the process. In relation to audio, feedback is used internal to many electronics as an integral part of the circuitry. However, "feedback" or "feedback loop" are often used to refer to the undesired continuous reamplification of a signal or portion of a signal in a loop as a result of the loop gain exceeding unity. "Feedback" is also often used to refer to the nasty squeal you hear when a microphone is placed too close to a speaker, a result of the electronic and acoustic system gain exceeding unity.