The energy barrier to chemical reaction. In vibrationally adiabatic transition-state theory its height is the zero-point energy of the activated complex minus that of the reactants.
Source:
PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 183 (https://doi.org/10.1351/pac199668010149)