In physics, equilibrium is the state of a system subjected to opposite forces that balance each other, so that the variables describing the system stop evolving and any move is stopped. In geography, what may rather be observed are stationary states, when equality of incoming and outgoing flows of an open system implies that there is no variation of the system (interactions, internal exchanges however remain). So, a region which receives as many immigrants from other regions as it sends emigrants will be said to be at equilibrium; a country’s balance of trade is said at equilibrium if importations are equal to exportations; in demography, a population is stationary if births over a given period equal deaths.
A balance is stable if the corresponding state restores itself after a small perturbation of the system, it is unstable in the case where the system state deviates from it after a perturbation.
In the frame of the spatial planning policy in France, the “métropoles d’équilibre” (balance metropolises), eight in number, have been designated in 1964 in order to host public facilities and investments with the aim to give back these cities greater autonomy with respect to Paris.
In economy, the theory of general equilibrium refers to the situation that is reached when global supply and demand are satisfied.