The atmospheric shell lying just above the troposphere which is characterized by an increasing temperature with altitude. The stratosphere begins at the tropopause (about 10-15 km height) and extends to a height of about 50 km, where the lapse rate changes sign at the stratopause and the beginning of the mesosphere.
Source:
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2217 (https://doi.org/10.1351/pac199062112167)