Oxidation-reduction potential (ORP) is an indicator of the oxidizing or reducing ability of a solution and is measured by methods similar to pH.

The ORP is measured using a pH meter with millivoltmeter function in conjunction with a noble metal electrode (platinum or gold) and a reference electrode. The redox potential is affected by temperature and must be measured at a constant temperature (e.g., 25 °C).

In other words, it is the energy level (electric potential) determined by the equilibrium conditions between oxidizing agents (Mz+) and reducing agents (M (z-n) +) coexisting in a solution.