equilibrium

In physical geography, the state of an open system where the inputs, throughputs, and outputs of energy or matter are in balance. For example, a river reach is in equilibrium when erosion and deposition are in balance through negative feedback; when surplus load is deposited, the morphology of the reach alters, increasing the capacity for transport and avoiding further deposition.

In static equilibrium there is no perceived change, in steady state equilibrium, fluctuation about a mean, in dynamic equilibrium, fluctuation about a moving average, and in dynamic metastable equilibrium, fluctuation about a moving average, marked by discontinuities.

In the humanities, neoclassical economists see the market mechanism as being a force which will re-establish an equilibrium, and define an equilibrium price as the price established by the interaction of supply and demand. Similarly, some economists and politicians have believed that market mechanisms can bring about spatial equilibrium—equality between regions, usually expressed in terms of wages. This balance, they believe, will be brought about by the interaction of labour moving to high wage areas and capital moving to regions where wage rates are low. In practice, this mobility is severely restricted: workers may not be able to sell their houses, and capitalists may not get planning permission to move into cheaper areas, for example.

Balance; more particularly, any state of affairs which no actor has an individual incentive to disturb. The study of political equilibria derives from two traditions: the tradition of balance of power in international relations; and the tradition of game theory.

In a balance of power, aggression is deterred because it would overall do more harm than good to the aggressor. Analogously, a game is in equilibrium if no actor would benefit from shifting his strategy to another of those available to him or her. Some equilibria are defective: a notorious case is simple Prisoners' Dilemma, in which each player would be better off if both moved from the equilibrium in which both defect, but neither has an incentive to do so.