Define the common ion effect and explain it with an example.

(N/A) Definition: The common ion effect is defined as the shift in equilibrium upon adding a substance that provides more of an ionic species already present in the dissociation equilibrium. This phenomenon is based on $Le \ Chatelier's$ principle.
Example: Consider the dissociation equilibrium of acetic acid $(CH_{3}COOH)$:
$CH_{3}COOH_{(aq)} \rightleftharpoons H_{(aq)}^{+} + CH_{3}COO_{(aq)}^{-}$
The ionization constant is given by $K_{a} = \frac{[H^{+}][CH_{3}COO^{-}]}{[CH_{3}COOH]}$.
If sodium acetate $(CH_{3}COONa)$ is added to this solution,it provides a high concentration of acetate ions $(CH_{3}COO^{-})$. According to $Le \ Chatelier's$ principle,the increase in the concentration of the product $(CH_{3}COO^{-})$ causes the equilibrium to shift to the left (towards the reactants) to minimize the effect of the added ions.
Consequently,the concentration of $H^{+}$ ions decreases,and the degree of dissociation of acetic acid is suppressed. This is the common ion effect.