The Collaborative International Dictionary of English v.0.48:

ozone \o"zone\ ([=o]"z[=o]n or [-o]*z[=o]n"), n. [Gr. 'o`zwn
   smelling, p. pr. of 'o`zein to smell. See Odor.] (Chem.)
   A colorless gaseous substance (O3) obtained (as by the
   silent discharge of electricity in oxygen) as an allotropic
   form of oxygen, containing three atoms in the molecule. It is
   a strong oxidizer, and probably exists in the air, though by
   the ordinary tests it is liable to be confused with certain
   other substances, as hydrogen dioxide, or certain oxides of
   nitrogen. It derives its name from its peculiar odor, which
   resembles that of weak chlorine.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ozone
    n 1: a colorless gas (O3) soluble in alkalis and cold water; a
         strong oxidizing agent; can be produced by electric
         discharge in oxygen or by the action of ultraviolet
         radiation on oxygen in the stratosphere (where it acts as a
         screen for ultraviolet radiation)