\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}.