ozone

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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.
• noun Informal Fresh, pure air.

from The Century Dictionary.

• noun A modification of oxygen, having increased chemical activity; a colorless gas having a peculiar odor like that of air which contains a trace of chlorin.
• noun In the gaseous state, ozone, if seen through a column of sufficient depth or under considerable pressure, has a blue color like that of ordinary oxygen, but more marked, and when liquefied appears dark blue. The liquid boils under atmospheric pressure at—100° C. (—159° F.). It is magnetic, and more soluble in water than ordinary oxygen. Animals breathing air which contains ozone in appreciable quantity present the phenomena of slow respiration, enfeebled circulation, lowering of bodily temperature, venous condition of the blood, and ultimately death. Air charged with ozone has been applied to the treatment of distilled spirits with a view to the removal of fusel-oil, but with only partial success.

from the GNU version of the Collaborative International Dictionary of English.

• noun (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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun chemistry An allotrope of oxygen (symbol O3) having three atoms in the molecule instead of the usual two; it is a blue gas, generated from oxygen by electrical discharge; it is poisonous and highly reactive, but in the upper atmosphere it protects life on Earth from ultraviolet radiation.
• noun Fresh air, especially that breathed at the seaside and smelling of seaweed.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun 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)