The boundary between the stratosphere and the mesosphere. It occurs at a height in the atmosphere of approximately 50 kilometers; however this depends on latitude. The atmosphere is characterized by a decrease in pressure with respect to increased altitude. More importantly, regions within the atmosphere like the troposphere, stratosphere, and mesosphere are distinguishable because of distinct temperature gradients with relatively well-defined starting and ending points. The stratopause is the highest portion of the stratosphere, with a temperature of approximately 0 degrees Celsius; the stratopause can also be described as the warmest region between the mesosphere and the stratosphere.
[Journal of the Atmospheric Sciences; v50; 3608-24; 1993.] [Journal of the Atmospheric Sciences; v50; 3928-38; 1993.]
Source: Atmospheric Chemistry Glossary