*hyponitrous*

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 ]