Mesosphere - In the atmosphere, the region immediately above the stratosphere and innemdiately below the thermosphere. The mesosphere begins about 50 kilometers high at the stratopause and ends about 80 kilometers high at the mesopause. The temperature in the mesosphere decreases sharply with increased altitude.
[Journal of the Atmospheric Sciences; v49n24; 2353-2371; 1992.]
[Introduction to Meteorology; F.W. Cole; page 7; 1980; John Wiley and Sons New York.] 
Atmospheric Chemistry Glossary

The atmospheric layer between the stratosphere and the thermosphere. Located at an average elevation between 50 and 80 km above the earth's surface.
WRCC Climate and Weather Terms Glossary

The atmospheric layer above the stratosphere, extending from about 50 to 85 kilometers altitude. The temperature generally decreases with altitude.
NASA Earth Observatory

The region of the Earth's atmosphere between the upper limit of the stratosphere (approximately 30 km altitude) and the lower limit of the thermosphere (approximately 80 km altitude). In the mesosphere, temperature decreases with height.