The pressure exerted at the earth's surface by the atmosphere. For reference purposes a standard atmosphere is defined as 760 torr or millimeters of mercury, or 760,000 microns.
The pressure (force per area) created by the weight of the atmosphere. At higher elevations, the atmospheric pressure is lower because there is less air.
Pressure of the air and atmosphere surrounding us which changes from day to day. It is equal to 14.7 psia.