1) A unit of pressure; the pressure that will support a column of mercury 760 mm high at 0 °C.
2) 760 torr, 101.325 kPa
3) The sum total of all the gases surrounding the Earth, extending several hundred kilometers above the surface in a mechanical mixture of various gases in fluid-like motion. The permanent constituents are molecular nitrogen; 78.1%, molecular oxygen; 20.9%, argon; 0.934%, and approximately 0.036% carbon dioxide. Various other components exist in trace amounts. Not to be under emphasized, these trace components are where the interesting atmospheric chemistry occurs. The atmosphere can also be artificially divided into layers. Example: the troposphere (the layer closest to the earth) and the stratosphere (the layer above the troposphere).
source: [Elements of Meteorology; Miller, Albert and Thompson, Jack; pp. 6-9;1970;Charles E Merrill; Ohio.] [ Climate Systems Modeling; Salby, Murry; Ed. Kevin E. Trenberth; pp. 53-115; 1992; Cambridge University Press; London.]