The Collaborative International Dictionary of English v.0.48:

Hyponitrous \Hy`po*ni"trous\, a. [Pref. hypo- + nitrous.]
   (Chem.)
   Containing or derived from nitrogen having a lower valence
   than in nitrous compounds.
   [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hyponitrous acid
    n 1: an explosive white crystalline weak acid (H2N2O2)