Potential energy gap between reactants and the transition state, sometimes involving the zero point energies, but usually not.
See also: critical energy
Green Book, 2nd ed., p. 56 (https://dev.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 189 (https://doi.org/10.1351/pac199668010149)