The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hypophosphorous acid
    n 1: a clear or yellow monobasic acid (H3PO2) [syn:
         hypophosphorous acid, phosphorous acid,
         orthophosphorous acid]