hypophosphoric acid

Wiktionary

1. n. The oxyacid of phosphorus H4P2O6

GNU Webster's 1913

1. adj. an acid, P2H4O6, produced by the slow oxidation of moist phosphorus, and isolated only as a solution in water. It is regarded as a condensation product of one molecule of phosphoric acid with one of phosphorous acid, by partial dehydration.

WordNet 3.0

1. n. a crystalline tetrabasic acid (H4P2O6)