Source : Webster's Revised Unabridged Dictionary (1913)

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.

   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.

   {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}.

Source : WordNet®

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