In chemistry, biochemistry, and pharmacology, a dissociation constant (K) is a specific type of equilibrium constant that measures the propensity of a larger object to separate (dissociate) reversibly into smaller components, as when a complex falls apart into its component molecules, or when a salt splits up into its component ions.  The dissociation constant is the inverse of the association constant. In the special case of salts, the dissociation constant can also be called an ionization constant.
For a general reaction:
in which a complex  breaks down into x&thinsp;A subunits and y&thinsp;B subunits, the dissociation constant is defined as
where [A], [B], and [A&thinsp;B] are the equilibrium concentrations of A, B, and the complex A&thinsp;B, respectively.
One reason for the popularity of the dissociation constant in biochemistry and pharmacology is that in the frequently encountered case where x = y = 1, K has a simple physical interpretation: when [A] = K, then [B] = [AB] or, equivalently, .  That is, K, which has the dimensions of concentration, equals the concentration of free A at which half of the total molecules of B are associated with A.  This simple interpretation does not apply for higher values of x or y. It also presumes the absence of competing reactions, though the derivation can be extended to explicitly allow for and describe competitive binding.  It is useful as a quick description of the binding of a substance, in the same way that EC and IC describe the biological activities of substances.