Definition
The force exerted on a surface by the weight of the air above it. At sea level under standard conditions, air pressure is approximately 14.7 pounds per square inch (psi), or 29.92 inches of mercury (inHg), or 1013.2 millibars (hPa). Air pressure decreases with altitude because there is less air mass above any given point as you climb.
Plain English
The push that the air around you puts on everything it touches. Air has weight, and the column of air above you presses down on the ground, on your aircraft, and on you. The higher you go, the less air is above you, so the pressure drops.
Context Anchor
Seen in weather, altimeter setting, aircraft performance, and atmospheric pressure discussions.
Derivation
From Latin 'pressura' (a pressing, a squeezing), from 'premere' (to press). The word captures exactly what is happening: the atmosphere is literally pressing on everything inside it.
Why Pilots Care
Directly affects altimeter readings, engine performance, and lift at different altitudes.
Analogy
Think of air as a deep ocean above you. Near the bottom, more air is stacked overhead, so the pressure is higher; higher up, less air is above you, so the pressure is lower.
Grounding Statement
Imagine a tall column of air stretching from the ground up to the edge of space, sitting on a one-square-inch patch at sea level. That column weighs about 14.7 pounds -- and that weight is what air pressure feels.
Intuition Check
Air pressure does not mean wind. Wind is air moving sideways; air pressure is air pressing on an area, even when the air feels still.
Example Sentence 1
Before takeoff, the pilot dialed in the current air pressure of 30.05 inHg so the altimeter would read field elevation correctly.
Example Sentence 2
As the airplane climbed, lower air pressure reduced both lift and engine power output.