From 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]
From The Collaborative International Dictionary of English v.0.48:
Isonitroso- \I`so*ni*tro"so-\ [Iso- + nitroso-.] (Chem.)
A combining form (also used adjectively), signifying:
Pertaining to, or designating, the characteristic,
nitrogenous radical, NOH, called the isonitroso group.
[1913 Webster]