ORP is a term bandied about in the water ionizer scene – usually by wet behind the ears water ionizer salesmen.
Yet it is an important indicator of the ability of a water to kill bacteria (when an oxidant) and act as an antioxidant at the other end of the scale of millivolts. We’ve found over the years that ORP meters are notoriously unreliable. At one stage we used 3 at a time to get a good average reading.
Rather than spend a lot of time recapping on ORP, here’s a good descriptive report by Myron corporation, the water testing equipment people on the subject. If you study it and absorb all of you’ll know more than me!
I think in the next few years you will hear less about ORP and more and more about molecular hydrogen.