What is the boundary between the troposphere and stratosphere called?

The boundary between the troposphere and stratosphere is called the tropopause.

The tropopause is identified by a change in the usual temperature pattern with height. In the troposphere, temperature generally decreases as altitude increases. In the stratosphere, temperature eventually begins increasing with height because ozone absorbs ultraviolet radiation. The transition between these patterns marks the tropopause.

Its altitude is not constant. It is generally higher in the tropics, where it can reach roughly 17 kilometres, and lower near the poles, where it may be about 8 kilometres. Seasonal and weather-related changes also affect its height.

Most clouds and everyday weather occur below the tropopause, although powerful thunderstorms can briefly overshoot it. Jet streams flow near this boundary because strong temperature contrasts and changes in atmospheric density help create fast upper-level winds.