coefficient in front of the exponential factor expressing the empirical temperature dependence of the rate coefficient, k, on temperature, T,k = A exp^(-Ea/R.T), where Ea is the activation energy.
Sources:
Green Book, 2nd ed., p. 56 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 177 (https://doi.org/10.1351/pac199668010149)