median narcotic dose (ND 50)

https://dx.doi.org/10.1351/goldbook.M03813
Statistically derived dose of a substance expected to cause narcosis in 50% of test animals 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