a unit of pressure: the pressure that will support a column of mercury 760 mm high at sea level and 0 degrees centigrade

The accepted normal atmospheric pressure at sea level, equal to 14.696 pounds per square inch.

a hypothetical vertical distribution of atmospheric temperature, pressure, and density

A hypothetical vertical distribution of temperature, humidity, and pressure taken to be representative. Used for aircraft altimetry, engineering design, etc. The particular standard atmosphere used is the U. S. S. A., equivalent to the ICAO standard atmosphere (Reference 15).

a unit of pressure equal to 760 mm Hg

A standard atmosphere has been defined by the International Civil Aeronautical Organization (ICAO). It assumes a mean sea level temperature of 15°C a standard sea level pressure of 1,013.25 millibars or 29.92 inches of mercury, and a temperature lapse rate of 0.65°C per 100 meters up to 11 kilometers in the atmosphere.