The transmission of a signal from a later to an earlier stage. As a control mechanism, it acts on actual rather than expected performance. Cf.
feedforward.
Negative feedback multiplies the input by a factor smaller than one (typically an exponential function of the difference between actual output and some ideal). Systems exhibiting this property thus tend to self-stabilize after disturbances. In biological systems, negative feedback is the basis of homeostasis.
Positive feedback involves a multiplier larger than one, and serves to amplify deviations. Such runaway systems are often brought under control by negative feedback; this usually results in the output following a sigmoidal growth curve.
Higher order feedback is feedback based on feedback. Second order negative feedback leads to sinusoidal oscillations about an equilibrium unless damped (by first order negative feedback).
Feedback may be continuous (eg. with an autopilot. Such a device is called a servomechanism) or discrete (eg. with a thermostat switching the heat on and off). It may also be intrinsic (the system itself adjusts its input according to its output) or extrinsic (the system's environment receives the system's output and thereby affects the system's input).
Feedback may be instantaneous or affected by delay (a pause until feedback occurs), lag (a gradual response to changes in feedback) or both. The latter three possibilities (and especially the latter) may cause negative feedback systems to become unstable, oscillating with ever-increasing violence, but may also allow a positive feedback system to restore equilibrium given appropriate conditions.