The American Heritage® Dictionary of the English Language, 4th Edition
n. The outermost region of a planet's atmosphere.
n. The outermost layer of the earth's atmosphere, lying above the thermosphere and extending thousands of kilometers into space, from which molecules having sufficient velocity can escape the earth's gravitation.
n. an extremely thin atmosphere, as on Saturn's moon Dione
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
n. the outermost atmospheric layer
Word Usage
"The observations are expected to yield new insights into how the magnetosphere "force field" works, as well as how thin the uppermost layer of Earth's atmosphere, called the exosphere, actually is, researchers said."