ORP is the term usually used in this hobby which mean Oxidation Reduction Potential. Which is usually used as a gauge / measurement of the cleanliness of the water and it;s ability to break down contaminants in our tank water . Generally, the higher the ORP level, the greater it's ability the water is able to destroy contaminates in the water which is why it is often used with the Ozonizer and ORP controller to prevent overdose which might cause tank causality.