Common Ion Effect
Definition
Adding an ion that is already in a dissolution equilibrium pushes the solid back out: solubility drops.
Technical definition
The common ion effect is the solubility decrease of a sparingly soluble salt when a soluble salt containing a common ion is added. By Le Chatelier's principle the dissolution equilibrium shifts left, lowering the salt's solubility.
Example
AgCl dissolves less in 0.1 M NaCl solution than in pure water, because the extra Cl- from NaCl suppresses AgCl's dissolution.
Common confusion
The common ion must actually be part of the equilibrium. Adding NaNO3 to AgCl does nothing, but adding NaCl, which supplies the common Cl-, cuts solubility dramatically.