From The Collaborative International Dictionary of English v.0.44 [gcide]:
Halogen \Hal"o*gen\ (h[a^]l"[-o]*j[e^]n), noun [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}.
From WordNet (r) 2.0 [wn]:
noun
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
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