from The American Heritage® Dictionary of the English Language, 4th Edition
- n. The outermost shell of the atmosphere, between the mesosphere and outer space, where temperatures increase steadily with altitude.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. layer of the Earth's atmosphere directly above the mesosphere and directly below the exosphere
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. the atmospheric layer between the mesosphere and the exosphere
thermo- + -sphere (Wiktionary)