The dissociation or ionization of a weak acid, HA, can be represented as:
HA (aq) + H2O (l) ⇌ H3O+ (aq) + A- (aq)
This reaction is referred to as ionization equilibrium and is characterized by an equilibrium constant. The concentration of a pure liquid or a solid is taken as 1.
Ka is called acid dissociation constant or ionization constant of the acid.
The magnitude of the equilibrium constant is a measure of the strength of the acid. Higher the value of the equilibrium constant the stronger is the acid.