The binding constant is a special case of the equilibrium constant . It is associated with the binding and unbinding reaction of receptor (R) and ligand (L) molecules, which is formalized as:
.

The reaction is characterized by the on-rate constant and the off-rate constant , which have units of 1/(concentration time) and 1/time, respectively. In equilibrium, the forward binding transition should be balanced by the backward unbinding transition . That is,
,

where , and represent the concentration of unbound free receptors, the concentration of unbound free ligand and the concentration of receptor-ligand complexes. The binding constant, or the association constant is defined by
.

An often considered quantity is the dissociation constant , which has the unit of concentration. For the binding of receptor and ligand molecules in solution, the molar Gibbs free energy

Gibbs free energy

In thermodynamics, the Gibbs free energy is a thermodynamic potential that measures the "useful" or process-initiating work obtainable from a thermodynamic system at a constant temperature and pressure...

 , or the binding affinity is related to the dissociation constant via
,

in which is the ideal gas constant, temperature and the standard reference concentration = 1 mol / L.