halogen

noun

• any of the elements fluorine, chlorine, bromine, iodine, and astatine, occupying group VIIA (17) of the periodic table. They are reactive non-metallic elements which 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 vapour of iodine or another halogen:a halogen bulb