chlorine, Cl, atomic number 17 - a common nonmetallic element belonging to the halogens; best known as a heavy yellow irritating toxic gas; used to purify water and as a bleaching agent and disinfectant; occurs naturally only as a salt (as in sea water) --1is a kind ofchemical element, element; gas; halogen --1is a substance of sodium chloride, common salt --1has particulars: radiochlorine