Atmospheric pressure
The weight of the column of air above any point, at sea level ≈ 101.325 kPa or 760 mm Hg.
Definition
Atmospheric pressure is the force per unit area exerted by the weight of the air column above a given point. At sea level the column is about 10 metres-equivalent of water — roughly 101,325 Pa, or one 'atmosphere.' It falls exponentially with altitude, halving roughly every 5.5 km.
Torricelli first measured it in 1644 with a mercury barometer: a glass tube sealed at the top, inverted into a reservoir of mercury, leaves a column about 760 mm high, supported by atmospheric pressure pushing down on the reservoir. Pascal confirmed the atmospheric origin of the pressure in 1648 by sending his brother-in-law up the Puy-de-Dôme and watching the column shrink with altitude.
History
First measured by Evangelista Torricelli in 1644; altitude dependence verified by Blaise Pascal's Puy-de-Dôme experiment in 1648.