The height above ground level at which there is change in sign of the normal temperature 'lapse rate', dT/dz (the rate of change of the temperature with height). Several temperature inversions may be present in the air over a given site at different altitudes as result of various meteorological factors.
Source:
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2197 (https://doi.org/10.1351/pac199062112167)