The Collaborative International Dictionary of English v.0.48:

Hypophosphoric \Hy`po*phos*phor"ic\, a. [Pref. hypo- +
   phosphoric.] (Chem.)
   Pertaining to, or derived from, or containing, phosphorus in
   a lower state of oxidation than in phosphoric compounds; as,
   hypophosphoric acid.
   [1913 Webster]

   Hypophosphoric acid (Chem.), 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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hypophosphoric acid
    n 1: a crystalline tetrabasic acid (H4P2O6)