halogen definition
Freebase(0.00 / 0 votes)Rate this definition: Halogen. The halogens or halogen elements are a group in the periodic table consisting of five chemically related elements, fluorine, chlorine, bromine, iodine, and astatine. The artificially created element 117 may also be a halogen. In the modern IUPAC nomenclature, this group is known as group 17. The group of halogens is the only periodic table group which contains elements in all three familiar states of matter at standard temperature and pressure.