According to the U.S. NIOSH, the maximum exposure concentration from which one could escape within \(\pu{30 min}\) without any escape-impairing symptoms or any irreversible health effects.
Source: PAC, 2007, 79, 1153. (Glossary of terms used in toxicology, 2nd edition (IUPAC Recommendations 2007)) on page 1241 [Terms] [Paper]