metaphosphoric acid

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

• noun A colorless glassy polymeric phosphoric acid, (HPO₃)_n, that is highly deliquescent and forms phosphoric acid upon addition of water, used as a dehydrating agent, phosphorylating agent, and in dental cements.

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

• noun chemistry an anhydrous form of phosphoric acid that exists as a polymeric glassy solid, (HPO3)n; used as a drying agent and in dental cement

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

• noun a glassy solid acid ([HPO3]n) often used as a dehydrating agent