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1. An unstable, poisonous allotrope of oxygen, O3, that is formed naturally in the ozone layer from atmospheric oxygen by electric discharge or exposure to ultraviolet radiation, also produced in the lower atmosphere by the photochemical reaction of certain pollutants. It is a highly reactive oxidizing agent used to deodorize air, purify water, and treat industrial wastes.
2. Informal Fresh, pure air.

[German Ozon, from Greek ozon, neuter present participle of ozein, to smell.]

o·zo′nic (ō-zō′nĭk, ō-zŏn′ĭk), o′zon′ous (ō′zō′nəs) adj.