These are treatments of reaction rates in which the system is assumed to remain on a single potential-energy (reaction) surface during the entire course of reaction, or to remain in a conserved internal state.
Source:
PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 152 (https://doi.org/10.1351/pac199668010149)