The efficiency of binding to a biomolecular target can be expressed by various different (but somewhat similar) quantitative descriptors, depending on the target and the activity assay performed.
Common descriptors for the assessment of inhibitors include the
IC50 (the half-maximal inhibitory concentration, i.e. the concentration of a molecule that results in a 50% decrease in observable response from a biomolecular process), or the
Ki (the inhibition constant that is related to the IC50 but takes into account the art of inhibition, i.e. competitive vs non-competitive inhibition).
Similarly, when the target is a receptor, efficacy of binding is expressed by the relative ability to produce a maximum response, but can also be referred to by the potency
EC50 (the half-maximal effective concentration, i.e. the concentration of a molecule that yields a 50% response on a dose/response curve).