From WebRef.org
The downward pressure, at any given point in the atmosphere, of the gases directly above that point. Average pressure globally at sea level is 1,013,000 dynes per centimeter squared or 760 torr. This is defined as one atmosphere.
[A Field Guide to the Atmosphere; Day, John and Schaefer, Vincent; 347; 1981; Houghton Mifflin; Boston.] [Ice Time; Levenson, Thomas; 69-70; 1989; Harper and Row; NY.]
Source: Atmospheric Chemistry Glossary