The Collaborative International Dictionary of English v.0.48:

Phosgene \Phos"gene\, a. [Gr. fw^s light + the root of
   gi`gnesqai to be born: cf. F. phosg[`e]ne.] (Old Chem.)
   Producing, or produced by, the action of light; -- used
   specifically to designate a gas also called carbonyl
   chloride. See phosgene, n., and Carbonyl.
   [1913 Webster + PJC]

The Collaborative International Dictionary of English v.0.48:

phosgene \phos"gene\ (f[o^]s"j[=e]n or f[o^]z"j[=e]n), n.
   (Chem.)
   A reactive chemical substance (COCl2), also called
   carbonyl choride, used in synthesis of numerous substances.
   In the First World War it was also used as a poisonous gas in
   combat.
   [PJC]

The Collaborative International Dictionary of English v.0.48:

Carbonyl \Car"bon*yl\, n. [Carbon + -yl.] (Chem.)
   The radical (=CO), occuring, always combined, in many
   compounds, as the aldehydes, the ketones, urea, carbonyl
   chloride, etc.
   [1913 Webster]

   Note: Though denoted by a formula identical with that of
         carbon monoxide, it is chemically distinct, as carbon
         seems to be divalent in carbon monoxide, but
         tetravalent in carbonyl compounds.
         [1913 Webster]

   Carbonyl chloride (Chem.), a colorless gas, COCl2, of
      offensive odor, and easily condensable to liquid. It is
      formed from chlorine and carbon monoxide, under the
      influence of light, and hence has been called phosgene,
      or phosgene gas; -- called also carbon oxychloride. It
      is used in chemical synthesis, and was also used as a
      poison gas in World War I.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

phosgene
    n 1: a colorless poisonous gas that smells like new-mown hay;
         used in chemical warfare