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