The term 'HALOGEN' refers to a group of elements in the periodic table, including fluorine and iodine. Fluorine and iodine are both non-metallic elements that are highly reactive and form compounds with other elements. Fluorine is the most reactive of the halogens, while iodine is the least reactive. Both elements are used in a variety of industrial and medical applications, such as water purification, pharmaceuticals, and electronics.