Plain English Meaning
A high atmospheric layer well above where airplanes fly. It sits above the stratosphere and is the coldest part of the atmosphere.
Full Definition
The third layer of Earth's atmosphere, located above the stratosphere and below the thermosphere, extending roughly from 50 km (about 31 miles) to 85 km (about 53 miles) above the surface. Temperature decreases with altitude through this layer, reaching the coldest point in the atmosphere near its upper boundary.
Context Anchor
Seen in atmospheric-layer discussions in the Instrument Flying Handbook, especially when placing aviation weather and instrument flying in the lower part of the atmosphere.
Derivation
From Greek 'mesos' meaning 'middle,' plus 'sphere.' It is the middle layer in the standard atmospheric stack of troposphere, stratosphere, mesosphere, thermosphere, and .