the gaseous envelope surrounding the earth; the air.
2.
this medium at a given place.
3.
Astronomy. the gaseous envelope surrounding a heavenly body.
4.
Chemistry. any gaseous envelope or medium.
5.
a conventional unit of pressure, the normal pressure of the air at sea level, about 14.7 pounds per square inch (101.3 kilopascals), equal to the pressure exerted by a column of mercury 29.92 inches (760 mm) high.