Plain English Meaning
The very top edge of Earth's atmosphere, where the air is so thin it slowly thins out into outer space.
Full Definition
The outermost layer of Earth's atmosphere, beginning at roughly 440 miles (700 km) above the surface and extending until it gradually fades into space. Air molecules here are so sparse that they rarely collide with one another, and the layer has no clearly defined upper boundary.
Context Anchor
Seen in instrument flying discussions of atmospheric layers; it is background knowledge, not a layer a pilot operates in.
Derivation
From the Greek 'exo' meaning 'outside' and 'sphaira' meaning 'sphere' or 'ball.' Literally the 'outside sphere' — the outermost shell of atmosphere wrapped around the Earth.
Why Pilots Care
Pilots do not fly in the exosphere, but knowing where it fits helps keep the atmosphere’s layers in order and reinforces that air becomes with .