Source : Webster's Revised Unabridged Dictionary (1913)

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.

   {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.

Source : WordNet®

hypophosphoric acid
     n : a crystalline tetrabasic acid (H4P2O6)