Definition of halogen noun from the Oxford Advanced American Dictionary

 

halogen

 noun
noun
NAmE//ˈhælədʒən//
 
(chemistry)
 
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any of a set of five chemical elements, including fluorine, chlorine, and iodine, that react with hydrogen to form acids from which simple salts can be made. Halogens, in the form of gas, are used in lamps and stoves.
See the Oxford Advanced Learner's Dictionary entry: halogen