Feedback describes the situation when output from (or information

Information

Information in its most restricted technical sense is a message or collection of messages that consists of an ordered sequence of symbols, or it is the meaning that can be interpreted from such a message or collection of messages. Information can be recorded or transmitted. It can be recorded as...

 about the result of) an event or phenomenon

Phenomenon

A phenomenon , plural phenomena, is any observable occurrence. Phenomena are often, but not always, understood as 'appearances' or 'experiences'...

 in the past will influence an occurrence or occurrences of the same (i.e. same defined) event / phenomenon (or the continuation / development of the original phenomenon) in the present or future.
When an event is part of a chain of cause-and-effect

Causality

Causality is the relationship between an event and a second event , where the second event is understood as a consequence of the first....

 that forms a circuit or loop, then the event is said to "feed back" into itself.

Feedback is also a synonym for:

• Feedback signal - the information about the initial event that is the basis for subsequent modification of the event
• Feedback loop - the causal path that leads from the initial generation of the feedback signal to the subsequent modification of the event
• Audio feedback
  Audio feedback
  Audio feedback is a special kind of positive feedback which occurs when a sound loop exists between an audio input and an audio output...
  
  - the special kind of positive feedback
  Positive feedback
  Positive feedback is a process in which the effects of a small disturbance on a system include an increase in the magnitude of the perturbation. That is, A produces more of B which in turn produces more of A. In contrast, a system that responds to a perturbation in a way that reduces its effect is...
  
   that occurs when a loop exists between an audio input and output.

Feedback is a mechanism, process or signal that is looped back to control a system

System

System is a set of interacting or interdependent components forming an integrated whole....

 within itself.