KB defines the equilibrium dissociation constant for a competitive antagonist: i.e. the molar concentration that would occupy 50% of the receptors at equilibrium.
Kd defines the dissociation constant for a radiolabeled ligand determined by saturation binding analysis. It is the molar concentration of radioligand which, at equilibrium, occupies 50% of the receptors.
Ki defines the inhibition constant for a ligand, which denotes the affinity of the ligand for a receptor. Measured using a radioligand competition binding assay, it is the molar concentration of the competing ligand that would occupy 50% of the receptors if no radioligand was present. It is calculated from the IC50 value using the Cheng-Prusoff equation.