The binding (association) constant, Ka, is equal to [product]/[reactants]. You'll notice it's essentially Keq. The dissociation constant, Kd, is the inverse of Ka. Binding/dissociation constants are frequently used to describe receptor/ligand binding interaction, as you can see from the question.
Do not confuse Ka with the acid dissociation constant (also Ka), which aims to measure the deprotonation of an acid at equilibrium.