<unk>  is the<unk>  loop system in which the system responds to perturbation in the same direction as the perturbation (It is sometimes referred to as cumulative causation). In contrast, a system that responds to the perturbation in the opposite direction is called a negative<unk>  system. These concepts were first recognized as broadly applicable by Norbert Wiener in his 1948 work on Cybernetics.* 
A system in equilibrium in which there is<unk>  to any change in its current state is said to be in an unstable equilibrium, whereas one with negative<unk>  is said to be in a stable equilibrium.
The end result of a<unk>  is often amplifying and "explosive", i.e. a small perturbation results in big changes.  This<unk> , in turn, will drive the system further away from its original setpoint, thus amplifying the original perturbation signal, and eventually become explosive because the amplification often grows exponentially (with the first order<unk> , or even hyperbolically  (with the second order<unk> . Indeed, chemical and nuclear fission based explosives offer an excellent physical demonstration of<unk> . Bombarding fissile material with neutrons causes it to emit even more neutrons, which in turn affect the material. The greater the mass of fissile material, the larger the amplification, resulting in greater<unk> . If the amplification is great enough,  the process accelerates until the fissile material is spent or dispersed by the resulting explosion.
Both<unk>  and negative<unk>  are closed systems, because the system is closed by a<unk>  loop, i.e. the response of the system depends on the<unk>  signal to complete its function; without such a loop, it would become an open system.  In contrast, a feed-forward system is an "open system" since it does not have any<unk>  loop, and does not rely on<unk>  signal to function.
Examples of<unk>  and negative<unk> , open and closed systems can be found in ecological, biological, social systems and in engineering control systems such as servo control systems.
Positive feedback
Feedback
Goal-directed therapy
Mind–body intervention
Patient engagement
Patient education
Chaos theory
Affect control theory