NO2

from The Collaborative International Dictionary of English v.0.48

Nitric \Ni"tric\, a. [Cf. F. nitrique. See {Niter}.] (Chem.)
   Of, pertaining to, or containing, nitrogen; specifically,
   designating any one of those compounds in which, as
   contrasted with {nitrous} compounds, the element has a higher
   valence; as, nitric oxide; nitric acid.
   [1913 Webster]

   {Nitric acid}, a colorless or yellowish liquid obtained by
      distilling a nitrate with sulphuric acid. It is powerfully
      corrosive, being a strong acid, and in decomposition a
      strong oxidizer.

   {Nitric anhydride}, a white crystalline oxide of nitrogen
      ({N2O5}), called {nitric pentoxide}, and regarded as the
      anhydride of nitric acid.

   {Nitric oxide}, a colorless poisous gas ({NO}) obtained by
      treating nitric acid with copper. On contact with the air
      or with oxygen, it becomes reddish brown from the
      formation of nitrogen dioxide ({NO2}, also called nitric
      dioxide or nitric peroxide).
      [1913 Webster]

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

Nitrite \Ni"trite\, n. [Cf. F. nitrite. See {Niter}.] (Chem.)
   A salt or ester of nitrous acid; a compound bearing the
   {-NO2} radical.
   [1913 Webster +PJC]

   {Amyl nitrite}, a yellow oily volatile liquid, used in
      medicine as a depressant and a vaso-dilator. Its
      inhalation produces an instantaneous flushing of the face.
      [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]

from The Collaborative International Dictionary of English v.0.48

Chromophore \Chro"mo*phore\, n. [Gr. ? color + fe`rein to bear.]
   (Chem.)
   Any chemical group or residue (as {NO2}; {N2}; or {O2}) which
   imparts some decided color to the compound of which it is an
   ingredient.
   [1913 Webster]