halogen

noun

• any of the elements fluorine, chlorine, bromine, iodine, and astatine, occupying group VIIA (17) of the periodic table. They are reactive nonmetallic elements that form strongly acidic compounds with hydrogen, from which simple salts can be made.
• [as modifier] denoting lamps and radiant heat sources using a filament surrounded by the vapor of iodine or another halogen:halogen headlights