The Collaborative International Dictionary of English v.0.48:

Cyanogen \Cy*an"o*gen\ (s?-?n"?-j?n), n. [Gr. ky`anos a dark
   blue substance + -gen: cf. F. cyanog[`e]ne. So called because
   it produced blue dyes.] (Chem.)
   A colorless, inflammable, poisonous gas, C2N2, with a
   peach-blossom odor, so called from its tendency to form blue
   compounds; obtained by heating ammonium oxalate, mercuric
   cyanide, etc. It is obtained in combination, forming an
   alkaline cyanide when nitrogen or a nitrogenous compound is
   strongly ignited with carbon and soda or potash. It conducts
   itself like a member of the halogen group of elements, and
   shows a tendency to form complex compounds. The name is also
   applied to the univalent radical, CN (the half molecule of
   cyanogen proper), which was one of the first compound
   radicals recognized.
   [1913 Webster]

   Note: Cyanogen is found in the commercial substances,
         potassium cyanide, or prussiate of potash, yellow
         prussiate of potash, Prussian blue, Turnbull's blue,
         prussic acid, etc.
         [1913 Webster]

WordNet (r) 3.0 (2006):

cyanogen
    n 1: a colorless toxic gas with a pungent almond odor; has been
         used in chemical warfare