A path trajectory over a potential-energy (reaction) surface which corresponds to the outermost vibrational turning points for the molecular species involved. This path was suggested as a device for calculating the probability of quantum-mechanical tunnelling: the trajectory for the system is considered to follow this path.
Source:
PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 172 (https://doi.org/10.1351/pac199668010149)