Common Ion Effect
When a reaction has reached equilibrium, and an outside source adds more of one of the ions that is already in solution, the result is to cause the reverse reaction to occur at a faster rate and reestablish the equilibrium. This is called the common ion effect. For example, in this equilibrium reaction:
NaCl <---> Na(+) + Cl(-)
the addition of concentrated HCl (12M) adds H(+) and Cl(-) both at a concentration of 12 M. This increases the concentration of the Cl(-) and disturbs the equilibrium. The reaction will shift to the left and cause some solid NaCl to come out of solution.
The "common" ion is the one already present in an equilibrium before a substance is added that increases the concentration of that ion. The effect is to reverse the solution reaction and to decrease the solubility of the original substance, as shown in the above example