An acid dissociation constant, K_a, (also known as acidity constant, or acid-ionization constant) is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions. In aqueous solution, the equilibrium of acid dissociation can be written symbolically as:

where HA is a generic acid that dissociates into A^-, known as the conjugate base of the acid and a hydrogen ion which combines with a water molecule to make an hydronium ion. In the example shown in the figure, HA represents acetic acid, and A^- represents the acetate ion, the conjugate base.