The Collaborative International Dictionary of English v.0.48:

Atmo \At"mo\, n. [Contr. fr. atmosphere.] (Physics)
   The standard atmospheric pressure used in certain physical
   measurements calculations; conventionally, that pressure
   under which the barometer stands at 760 millimeters, at a
   temperature of 0[deg] Centigrade, at the level of the sea,
   and in the latitude of Paris. --Sir W. Thomson.
   [1913 Webster] Atmologic