hypophosphorous acid

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A clear, colorless or slightly yellow liquid, H₃PO₂, used as a reducing agent.

Wiktionary

1. n. chemistry A pale yellow liquid, H3PO2, used as a reducing agent.

GNU Webster's 1913

1. adj. (Chem.) , an acid, H3PO2, whose salts are produced by the action of barium hygrate on phosphorus. It may be obtained from its water solution, by exaporation and freezing, as a white crystalline substance. It is a powerful reducing agent.

WordNet 3.0

1. n. a clear or yellow monobasic acid (H3PO2)