Half-maximal inhibitory concentration (IC50)

The concentration of a drug or inhibitor needed to inhibit a biological process or response by 50%.

IC50 is commonly used as a measure of drug potency in whole cell assays. IC50 assays are also used for screening in target-based drug discovery campaigns.

Tool for Prediction of pIC50 values (negative log of the IC50 value) from the IC50 values (Sanjeev’s Lab, 2016)

50% of what? How exactly are IC50 and EC50 defined? (GraphPad, 2010)