What is Atmospheric Pressure?

Legal Definition
Atmospheric pressure, sometimes also called barometric pressure, is the pressure exerted by the weight of air in the atmosphere of Earth (or that of another planet). In most circumstances atmospheric pressure is closely approximated by the hydrostatic pressure caused by the weight of air above the measurement point. Low-pressure areas have less atmospheric mass above their location, whereas high-pressure areas have more atmospheric mass above their location. Likewise, as elevation increases, there is less overlying atmospheric mass, so that atmospheric pressure decreases with increasing elevation. On average, a column of air one square centimetre [cm2] (0.16 sq in) in cross-section, measured from sea level to the top of the atmosphere, has a mass of about 1.03 kilograms (2.3 lb) and weight of about 10.1 newtons (2.3 lbf). That force (across one square centimeter) is a pressure of 10.1 N/cm2 or 101,000 N/m2. A column 1 square inch (6.5 cm2) in cross-section would have a weight of about 14.7 lb (6.7 kg) or about 65.4 N.
-- Wikipedia
Legal Definition
The measurement of 14.7 PSI, 101330 pascals, 29.92 inches, 1013.25 mb, and 1.03 atmosphere. AKA barometric pressure.