met·a·phos·phor·ic acid 
(mĕt′ə-fŏs-fôrĭk)
(mĕt′ə-fŏs-fôrĭk)n.
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.