hypophosphorous

from The Century Dictionary.

• In chem., containing less oxygen than phosphorous acid contains.

from the GNU version of the Collaborative International Dictionary of English.

• adjective (Chem.) Pertaining to, or containing, phosphorus in a lower state of oxidation than in phosphoric compounds.
• adjective (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.

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

• adjective Of or pertaining to hypophosphorous acid or its derivatives