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changes.mady.by.user Stephen R Wilk

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Excerpt

A stable position in which all forces are balanced (vector sum is zero) and the object of interest is not in motion..  Equilibria may be stable or unstable depending on whether the force acts back toward or away from the equilibrium position if the particle is slightly displaced from in.

Since the force at the equilibrium position is zero, the potential energy curve at the equilibrium will be horizontal.  This will be a local minimum or maximum, but is not necessarily global.  Hence there can be several equilibria as a particle moves along a line. 

Note that the net force must be zero.  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 from its equilibrium length 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

Add figures of force vs. displacement and potential energy vs. displacement.