https://doi.org/10.1351/goldbook.P04787
The temperature that a dry air parcel would have if lowered or raised adiabatically to a level of 1000 \(\text{mbar}\) pressure (or other arbitrary @S05921@).
Source:
PAC, 1990, 62, 2167. (Glossary of atmospheric chemistry terms (Recommendations 1990)) on page 2207 [Terms] [Paper]