Real Dictionary

halogen
A halogen is one of a group of chemical elements that includes chlorine, fluorine, and iodine. Halogens are often used in lighting and heating devices.

halogen in American English
any of the five very reactive, nonmetallic chemical elements making up group VIIA of the periodic table: fluorine is the smallest and most reactive of the group, and astatine is the largest and least reactive
noun: any of the electronegative elements, fluorine, chlorine, iodine, bromine, and astatine, that form binary salts by direct union with metals

halogen in British English
noun: any of the chemical elements fluorine, chlorine, bromine, iodine, and astatine. They are all monovalent and readily form negative ions