Meaning of exosphere

ex•o•sphere

Pronunciation: (ek'sō-sfēr"), [key]
— n.
  1. the highest region of the atmosphere, where the air density is so low that a fast-moving air molecule is more than 50 percent likely to escape from the atmosphere instead of hitting other molecules.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.
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