That region of the Earth's atmosphere above the mesosphere where the neutral temperature increases with height. It begins at the mesopause at about 80-85 km and extends to the exosphere where the temperature regime becomes isothermal.

The atmospheric layer above the mesosphere. It extends from 90 km to outer space.
WRCC Climate and Weather Terms Glossary

The outermost shell of the atmosphere, between the mesosphere and outer space; where temperatures increase steadily with altitude.
NASA Earth Observatory

Thermosphere- The outermost shell of the atmosphere, between the mesosphere and outer space; where temperatures increase steadily with altitude. 
Atmospheric Chemistry Glossary