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}.