median lethal time (TL 50)

https://dx.doi.org/10.1351/goldbook.M03811
Statistically derived average time interval during which 50% of a given population may be expected to die following acute administration of a chemical or physical agent (radiation) at a given concentration under a defined set of conditions.
Source:
PAC, 1993, 65, 2003 (Glossary for chemists of terms used in toxicology (IUPAC Recommendations 1993)) on page 2068