The period of time that elapses between the start of the reaction and the attainment of a given extent of a reaction.
PAC, 1993, 65, 2291
(Nomenclature of kinetic methods of analysis (IUPAC Recommendations 1993))
on page 2297
IUPAC. Compendium of Chemical Terminology, 2nd ed. (the "Gold Book"). Compiled by A. D. McNaught and A. Wilkinson. Blackwell Scientific Publications, Oxford (1997). XML on-line corrected version: http://goldbook.iupac.org (2006-) created by M. Nic, J. Jirat, B. Kosata; updates compiled by A. Jenkins. ISBN 0-9678550-9-8. doi:10.1351/goldbook