Feedback is usually defined something like this: "The control of input as a function of output by returning a portion of the output to the input"1 or "the transfer of energy from the output to the input of a system, or from one part of a system to another in a direction opposite to the main flow of energy."2 Computer scientists may wish to substitute the word "information" for that of "energy."
Since any input must propagate in a system before an output results, there seems to be a definite temporal nature to the concept of feedback. For this reason I would like to offer my own definition for consideration:
Transfer (continuous or discrete) from one point to another such that future such transfer will be affected.
This definition also avoids the problem of tying the definition to "a portion of the output" or "the input." Freeing the definition from these constraints may make sense because a feedback "loop" may occur within the information flow boundaries of input and output.
1 McGraw-Hill dictionary of Physics and Mathematics, 1978, p. 353.
2 "Feedback," Van Nostrand's Scientific Encyclopedia, 1983, p. 1150.
3 Curtis D. Johnson, Process Control Instrumentation
Technology (Englewood Cliffs, NJ: Prentice Hall, 1988), p. 3.
of David Lee Winston Miller.