From The Collaborative International Dictionary of English v.0.48:
nitro- \ni"tro-\pref.
1. A combining form or an adjective denoting the presence of
niter.
[1913 Webster]
2. (Chem.) A combining form (used also adjectively)
designating certain compounds of nitrogen or of its acids;
as nitrohydrochloric, nitrocalcite; also, designating the
group or radical NO2, or its compounds, as nitrobenzene
(C6H5.NO2).
[1913 Webster]
Nitro group, the radical NO2; -- called also nitroxyl.
[1913 Webster]
From The Collaborative International Dictionary of English v.0.48:
Nitroxyl \Ni*trox"yl\, n. [Nitro- + oxygen + -yl.] (Chem.)
The group -NO2, usually called the nitro group.
[1913 Webster]