IC50
IC50 is the concentration of an inhibitor that knocks a measured response down to half its uninhibited level. It is a workhorse number for ranking how potent a blocker is: the lower the IC50, the less drug you need to achieve half-inhibition.
It is read off a dose–response curve, where increasing inhibitor concentration is plotted against the remaining signal — enzyme activity, ligand binding, cell viability, and so on. IC50 sits at the midpoint of that curve. Because it is defined relative to the specific readout, the same compound can have different IC50 values in different assays.
Crucially, IC50 is condition-dependent: for a competitive inhibitor it rises as substrate concentration rises, because more substrate out-competes the inhibitor. That is why an assay-independent Ki is preferred for comparing inhibitors across experiments, and why an IC50 should always be reported together with the assay conditions used to measure it.
An enzyme inhibitor with an IC50 of 50 nM halves the enzyme's activity at 50 nM under the tested conditions; a 5 nM inhibitor on the same target is ten-fold more potent.
IC50 marks half-maximal inhibition on a dose–response curve.
IC50 is for inhibitors (blocking a response); EC50 is the parallel measure for agonists (producing a response). Both mark the half-maximal point of their respective curves.