hpo3

from The Collaborative International Dictionary of English v.0.48

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}.
   [1913 Webster +PJC]