Pronunciation: (mez'u-pôz", mes'-, mē'zu-, -su-), [key]
1. the boundary or transition zone between the mesosphere and the ionosphere. Cf. mesosphere (def. 1).
2. the top of the mesosphere, determined by the appearance of a temperature minimum near an altitude of 50 mi. (80 km). Cf. mesosphere (def. 2).
Random House Unabridged Dictionary, Copyright © 1997, by Random House, Inc., on Infoplease.