The Collaborative International Dictionary of English v.0.48:

Halogen \Hal"o*gen\ (h[a^]l"[-o]*j[e^]n), n. [Gr. "a`ls, "alo`s,
   salt + -gen: cf. F. halog[`e]ne.] (Chem.)
   An electro-negative element or radical, which, by combination
   with a metal, forms a haloid salt; especially, chlorine,
   fluorine, bromine, and iodine; sometimes, also cyanogen. See
   Chlorine family, under Chlorine.
   [1913 Webster]

WordNet (r) 3.0 (2006):

halogen
    n 1: any of five related nonmetallic elements (fluorine or
         chlorine or bromine or iodine or astatine) that are all
         monovalent and readily form negative ions