Ozone (O3) is a substance consisting of molecules of three bound oxygen atoms. At standard temperature and pressure it is a blue gas. It forms a dark blue liquid below -112 C and a dark blue solid below -193 C.
It is a highly corrosive and poisonous substance and a common pollutant. It has a sharp, pungent odour. It's present in low concentrations throughout the Earth's atmosphere. It is formed naturally from O2 by electrical discharges, e.g., lightning, and by action of high energy electromagnetic radiation.
The highest levels of ozone in the atmosphere are in the stratosphere, in a region also known as the ozone layer. Here it filters out much ultraviolet light from the Sun that would be harmful to most forms of life. See also ozone hole.
Ozone was discovered by Christian Friedrich Schonbein in 1840.