The Collaborative International Dictionary of English v.0.48:

Pyrophosphoric \Pyr`o*phos*phor"ic\, a. [Pyro- + phosphoric.]
   (Chem.)
   Pertaining to, or designating, an acid, H4P2O7, which is
   obtained as a white crystalline substance. Its salts are
   obtained by heating the phosphates.
   [1913 Webster]