Nitro- \Ni"tro-\

1. A combining form or an adjective denoting the presence of niter.

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.

Nitro group, the radical NO2; -- called also nitroxyl.

Source: Webster's Revised Unabridged Dictionary (1913)