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The outermost layer of the atmosphere, between the mesosphere and interplanetary space, where air becomes extremely thin and temperatures increase steadily with altitude.
ther′mo·spher′ic (-sfîr′ĭk, -sfĕr′ĭk) adj.
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
(Physical Geography) pertaining to the thermosphere
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014