Plain English Meaning
The weight of the air pressing down on everything beneath it. The higher you go, the less air there is above you, so the pressure goes down.
Full Definition
The force per unit area exerted by the weight of the column of air above a given point in the atmosphere. At sea level under standard conditions, atmospheric pressure is 29.92 inches of mercury (inHg), or 1013.2 millibars (hPa), and it decreases with increasing altitude.
Context Anchor
Seen in weather, altimeter setting, altitude, and aircraft performance discussions.
Derivation
From the Greek atmos meaning or air, and the Latin pressura meaning a pressing. So literally, the pressing of the air — a useful reminder that air has weight and pushes on everything it touches.