pyrophosphoric acid

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A glacial or syrupy viscous liquid, H₄P₂O₇, used as a catalyst and in organic chemical manufacture.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A syrupy liquid formed by the dehydration of phosphoric acid; its salts, the pyrophosphates, are used in medicine.
• noun inorganic chemistry The acid formed by removing a molecule of water from two molecules of phosphoric acid, H4P2O7; this reaction slowly reverses in the presence of water.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a solid acid formed by reactions of orthophosphoric acid