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The Collaborative International Dictionary
nitro-

nitro- \ni"tro-\pref.

  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 ( C6H5.NO2).

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

Douglas Harper's Etymology Dictionary
nitro-

before vowels nitr-, word-forming element used scientifically and indicating "nitrogen, nitrate" or "nitric acid," from comb. form of Greek nitron (see nitre).

Wiktionary
nitro-

pre. 1 nitrate, nitrogen 2 containing the nitro functional group