Alkalinity, in place of carbonate hardness, or dKH.
Alkalinity refers to the pH of any substance north of 7.0. Carbonate hardness refers to the amount of carbonate disolved in solution. The two terms are not interchangeable, yet they are within our hobby with great regularity.
This one has bugged me for years (decades, actually).
That one has been the cause of much confusion. I can't tell you how many times I've asked someone "Where do you keep your alkalinity?" only to get the standard reply of 8.3 (their ph)...

It is a green acropora.





