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]