(1) A feedback in which the system responds to the perturbation in the same direction as the perturbation
(2) A feedback mechanism resulting in the amplification or growth of the output signal
A feedback mechanism is a loop system wherein the system responds to a perturbation. The response may be in the same direction (as in positive feedback) or in the opposite direction (as in negative feedback).
A positive feedback is one that which responds to the perturbation in the same direction as the perturbation. It tends to initiate or accelerate a biological process. In this system, the original perturbation signal is amplified, and the output can grow exponentially or even hyperbolically.
One example of biological positive feedback is at the onset of contractions in childbirth. When contraction occurs, oxytocin is released into the body stimulating more contractions. Thus, the result is an increased amplitude and frequency of contractions. Another example is during the process of blood clotting. When a tissue is injured, signal chemicals are released. These chemicals activate circulating platelets to release more of these chemicals to activate more platelets that are essential during the formation of a blood clot. Other examples of positive feedback are the generation of nerve signal and gene regulation.