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]

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]

WordNet (r) 3.0 (2006):

nitro group
    n 1: the group -NO3