What is "hpo3"

metaphosphoric \met`a*phos*phor"ic\, a. [Pref. meta- + phosphoric.] (Chem.) Pertaining to, or designating, a monobasic acid, HPO3, analogous to nitric acid, and, by heating phosphoric acid, obtained as a crystalline substance, commonly called glacial phosphoric acid. Compare the tribasic orthophosphoric acid.