Oxidation-reduction potential (ORP), also known as REDOX, is a measure of the ability of a molecule to oxidize or reduce another molecule.

Oxidation is the loss of electrons, so oxidizers accept electrons from other molecules. Reduction is the gain of electrons, so reducing agents donate electrons to other molecules.  The oxidation-reduction potential is measured as a single voltage in millivolts (mV). Oxidizing agents have a positive ORP value, while reducing agents have a negative ORP value.