Source : Webster's Revised Unabridged Dictionary (1913)

Halogen \Hal"o*gen\, 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,
   bromine, and iodine; sometimes, also, fluorine and cyanogen.
   See {Chlorine family}, under {Chlorine}.

Source : WordNet®

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