What's the "een" in Halloween?
A control system which monitors its effect on the system it is controlling and modifies its output accordingly. For example, a thermostat has two inputs: the desired temperature and the current temperature (the latter is the feedback). The output of the thermostat changes so as to try to equalise the two inputs.
Computer disk drives use feedback control to position the read/write heads accurately on a recording track. Complex systems such as the human body contain many feedback systems that interact with each other; the homeostasis mechanisms that control body temperature and acidity are good examples.