A stable position in which all forces are balanced and the object of interest is not in motion. This position need not be the lowest location, nor the position for which a spring is not extended – it depends upon the entire situation and the sum of all forces acting. In the case of the mass on a vertical spring the spring itself stretches to counter the effect of gravity. In the case of the mass between two springs both springs may be contracted or extended, so long as the force each exerts on the object is balanced by the force exerted by the other.
When an object is moved from its equilibrium position, it will usually move, since it is no longer in equilibrium. Usually the forces will tend to restore the object to its original position, but if displaced too far the object may seek a new equilibrium position.