Stable and Unstable Equilibrium
An equilibrium is said to be stable equilibrium when economic forces tend to push the market towards it. In other words, any divergence from the equilibrium position sets up forces, which tend to restore the equilibrium. This is the case in the market for good X illustrated.
At prices above Ope, there is an excess supply which pushes the price down. At prices below Ope there is an excess demand which pushes the price up.
Unstable equilibrium on the other hand is one such that any divergence from the equilibrium sets up forces which push the price further away from the equilibrium price. Consider the figure below which illustrates the market for good Y, which has a demand curve sloping upwards from left to right. Good Y might be an inferior good or a veblen good.
Price Ope is the equilibrium price and quantity Oqe is the equilibrium quantity. The "abnormal" demand curve means that at prices above Ope there is excess demand which pushes the price upwards and away from the equilibrium. Similarly, at prices below Ope, there is excess supply which pushes the prices even further down.
Thus, although equilibrium are states of rest at which no economic forces exist to change the situation, it is important to remember that not all equilibria are stable. The equilibrium in the figure above is sometimes called a knife edge equilibrium because a small change in price sends the system well away from equilibrium.