Binding affinity is used to characterize how strongly two molecules interact. Typically this will be a ligand and a receptor site, perhaps in a protein. Another common example would be a drug molecule binding to a receptor site somewhere in the brain. In simple cases where one drug molecule (D) binds to one receptor molecule (R) to form a complex (DR), the binding affinity can be described by the equilibrium constant:

The binding affinity can be looked at as a measure of how strongly a molecule binds to its receptor site. If a drug has higher affinity for its binding site, less of the drug will be required to achieve a response. Typically scientists designing pharmaceuticals would like a drug to have a high binding affinity for its receptor so that it can effect a strong response.