The Collaborative International Dictionary of English v.0.48:

Nitrous \Ni"trous\, a. [L. nitrosus full of natron: cf. F.
   nitreux. See Niter.]
   1. Of, pertaining to, or containing, niter; of the quality of
      niter, or resembling it.
      [1913 Webster]

   2. (Chem.) Of, pertaining to, or designating, any one of
      those compounds in which nitrogen has a relatively lower
      valence as contrasted with nitric compounds.
      [1913 Webster]

   Nitrous acid (Chem.), a hypothetical acid of nitrogen
      HNO2, not known in the free state, but forming a well
      known series of salts, viz., the nitrites.

   Nitrous oxide. See Laughing gas.
      [1913 Webster]

WordNet (r) 3.0 (2006):

nitrous acid
    n 1: an unstable inorganic acid known only in solution and as
         nitrite salts