half-life, \(\tau_{1/2}\)

of a transient entity

https://doi.org/10.1351/goldbook.HT07391

Time needed for a concentration of the entity to decrease to 1/2 of its original value.

Source:
PAC, 2007, 79, 293. 'Glossary of terms used in photochemistry, 3rd edition (IUPAC Recommendations 2006)' on page 349 (https://doi.org/10.1351/pac200779030293)