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]