Plain English Meaning
The high-altitude layer of the atmosphere where the air gets hotter the higher you go, because it absorbs strong radiation from the sun.
Full Definition
The fourth major layer of Earth's atmosphere, lying above the mesosphere and beginning at roughly 50 miles (80 km) above the surface. In this layer, temperature rises sharply with increasing altitude due to absorption of high-energy solar radiation by the very thin air.
Context Anchor
Seen in atmosphere-layer discussions in the Instrument Flying Handbook, especially when separating normal flying weather from the much higher layers of the atmosphere.
Derivation
From Greek 'thermos' meaning 'hot' and 'sphaira' meaning 'sphere' or '.' Literally the 'hot sphere' — , because in this layer can to thousands of degrees, even though the air is so you wouldn't the .