An empirical parameter characterizing the exponential temperature dependence of the rate coefficient, k, Ea = RT^2.(d(lnk)/dT), where R is the gas constant and T the thermodynamic temperature. The term is also used for threshold energies in electronic potential surfaces, in which case the term requires careful definition.
Source:
Green Book, 2nd ed., p. 55 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
See also:
PAC, 1993, 65, 2291. 'Nomenclature of kinetic methods of analysis (IUPAC Recommendations 1993)' on page 2294 (https://doi.org/10.1351/pac199365102291)
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1112 (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 151 (https://doi.org/10.1351/pac199668010149)