ozone - a compound whose molecules contain three oxygen atoms. Ozone
(O3) differs from oxygen gas
(O2), whose molecules contain two oxygen atoms. A gas, ozone in
the upper atmosphere it protects the earth from the sun's ultraviolet
radiation, but closer to the ground it can be polluting. Ozone is also used in water
treatment. Ozone in the upper atmosphere can be depleted by
chlorofluorocarbons.