Source : WordNet®

feedback
     n 1: the process in which part of the output of a system is
          returned to its input in order to regulate its further
          output
     2: response to an inquiry or experiment

Source : Free On-Line Dictionary of Computing

feedback
     
         Part of a system output presented at its input.
        Feedback may be unintended.  When used as a design feature,
        the output is usually transformed by passive components which
        attenuate it in some manner; the result is then presented at
        the system input.
     
        Feedback is positive or negative, depending on the sign with
        which a positive change in the original input reappears after
        transformation.  Negative feedback was invented by Black to
        stabilise {vacuum tube} amplifiers.  The behaviour becomes
        largely a function of the feedback transformation and only
        minimally a function of factors such as transistor gain which
        are imperfectly known.
     
        Positive feedback can lead to instability; it finds wide
        application in the construction of oscillators.
     
        Feedback can be used to control a system, as in {feedback
        control}.
     
        (1996-01-02)