Plain English Meaning
The layer of air around the Earth. It's what airplanes fly through, what we breathe, and what creates weather. It gets thinner the higher you go.
Full Definition
The envelope of gases surrounding the Earth, held in place by gravity, composed primarily of nitrogen (about 78%) and oxygen (about 21%), with smaller amounts of argon, carbon dioxide, water vapor, and trace gases. It extends from the surface upward, thinning gradually with altitude, and is the medium in which all aircraft fly.
Context Anchor
Seen in instrument flying, weather, altimeter, and aircraft performance discussions.