EC50
EC50 is the concentration of a drug that gets you halfway to its biggest possible effect — the midpoint on the way up. It is a compact way to say how much drug it takes to do a decent amount of work.
On a graded dose–response curve, especially when concentration is plotted on a logarithmic axis, the curve is S-shaped and the EC50 sits at its steep middle. A low EC50 means the drug is potent: a small amount already produces a substantial effect. Because it is read at the steepest part of the curve, EC50 is a reliable and reproducible measure of potency.
EC50 reflects potency, not the size of the maximum effect (that is Emax). Two drugs can share the same EC50 yet have very different ceilings, and a partial agonist's EC50 marks half of its own lower maximum, not half of a full agonist's. For doses given to whole organisms rather than concentrations in a dish, the analogous half-effect dose is the ED50.
If a vasodilator produces its maximal relaxation at high concentrations and half that relaxation at 10 nanomolar, its EC50 is 10 nanomolar.
The half-maximal-effect concentration.
EC50 is a functional, effect-based measure and is not the same as Kd, the binding-based affinity constant; receptor reserve can make EC50 much lower than Kd.