Pronunciation: (mez'u-sfēr", mes'-, mē'zu-, -su-), [key]
1. (in the classification of the earth's atmosphere by chemical properties) the region between the ionosphere and the exosphere, extending from about 250–650 mi. (400–1050 km) above the surface of the earth.
2. (in the classification of the earth's atmosphere by thermal properties) the region between the stratosphere and the thermosphere, extending from about 20–50 mi. (32–80 km) above the surface of the earth.
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.