Real Dictionary

exosphere in American English
the outermost portion of a planet's, esp. the earth's, atmosphere, consisting of a hot layer of light atoms often moving at escape velocity
noun: 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

exosphere in British English
noun: the outermost layer of the earth's atmosphere. It extends from about 400 km above the earth's surface