The tropopause is a boundary region in Earth's atmosphere between the troposphere and the stratosphere. Here the air ceases to cool at -56.5°C (-70°F), and the air becomes almost completely dry.

Most commercial aircraft are flown in the tropopause. To put it into even simpler words, the tropopause is the interferance between the troposphere and the stratosphere.

This short article about science can be made longer. You can help Wikipedia by adding to it.