Source : Webster's Revised Unabridged Dictionary (1913)

Hypophosphorous \Hy`po*phos"phor*ous\, a. [Pref. hypo- +
   phosphorous.] (Chem.)
   Pertaining to, or containing, phosphorus in a lower state of
   oxidation than in phosphoric compounds; as, hypophosphorous
   acid.

   {Hypophosphorous acid} (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.

Source : WordNet®

hypophosphorous acid
     n : a clear or yellow-colored monobasic acid (H3PO2) [syn: {phosphorous
         acid}, {orthophosphorous acid}]