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 said to exhibit negative feedback. These concepts were first recognized as broadly applicable by Norbert Wiener in his 1948 work on cybernetics.
Positive feedback tends to cause system instability. When there is more positive feedback than there are stabilizing tendencies, there will usually be exponential growth of any oscillations or divergences from equilibrium. System parameters will typically accelerate towards extreme values, which may damage or destroy the system, or may end with the system 'latched' into a new stable state. Positive feedback may be controlled by signals in the system being filtered, damped or limited, or it can be cancelled or reduced by adding negative feedback.