Halogen

from 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

from 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]