half life, \(t_{1/2}\)

https://doi.org/10.1351/goldbook.H02716
For a given reaction the half life t1/2 of a reactant is the time required for its concentration to reach a value that is the arithmetic mean of its initial and final (equilibrium) values. For a reactant that is entirely consumed it is the time taken for the reactant concentration to fall to one half its initial value: The half life of a reaction has meaning only in special cases:
  1. For a first-order reaction, the half life of the reactant may be called the half life of the reaction.
  2. For a reaction involving more than one reactant, with the concentrations of the reactants in their stoichiometric ratios, the half life of each reactant is the same, and may be called the half life of the reaction.
If the concentrations of reactants are not in their stoichiometric ratios, there are different half lives for different reactants, and one cannot speak of the half life of the reaction.
See also: lifetime
Sources:
Green Book, 2nd ed., p. 55 (https://dev.goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1119 (https://doi.org/10.1351/pac199466051077)
PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 167 (https://doi.org/10.1351/pac199668010149)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 975 (https://doi.org/10.1351/pac199668040957)