A surface, usually taken to be a hyperplane, constructed at right angles to the minimum-energy path on a potential-energy (reaction) surface. In conventional transition-state theory it passes through the highest point on the minimum-energy path. In generalized versions of transition-state theory the dividing surface can be at other positions; in variational transition-state theory the position of the dividing surface is varied so as to get a better estimate of the rate constant.
PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 163 (https://doi.org/10.1351/pac199668010149)