Source : Webster's Revised Unabridged Dictionary (1913)

Hyponitrous \Hy`po*ni"trous\, a. [Pref. hypo- + nitrous.]
   (Chem.)
   Containing or derived from nitrogen having a lower valence
   than in nitrous compounds.

   {Hyponitrous acid} (Chem.), an unstable nitrogen acid, {NOH},
      whose salts are produced by reduction of the nitrates,
      although the acid itself is not isolated in the free state
      except as a solution in water; -- called also {nitrosylic
      acid}.

Source : WordNet®

hyponitrous acid
     n : an explosive white crystalline weak acid (H2N2O2)