an atmospheric transition zone or shell located between the mesosphere and the thermosphere at an altitude of c. 80 to 85 km (c. 50 to 53 miles) in which temperatures begin to rise with increasing altitude
See mesopause in American Heritage Dictionary 4
(mĕzˈə-pôzˌ, mĕsˈ-)
noun
An atmospheric area about 80 kilometers (50 miles) above the earth's surface, forming the upper boundary of the mesosphere.