equilibrium

In physics, an unchanging condition in which an undisturbed system can remain indefinitely in a state of balance. In a static equilibrium, such as an object resting on the floor, there is no motion. In a dynamic equilibrium, in contrast, a steady state is maintained by constant, though opposing, changes. For example, in a sealed bottle half-full of water, the constancy of the water level is a result of molecules evaporating from the surface and condensing on to it at the same rate.