This expression refers to the region of a potential-energy (reaction) surface or hypersurface that corresponds to molecular configurations that are closer to those of the products than to those of the reactants.
Source:
PAC, 1996, 68, 149. 'A glossary of terms used in chemical kinetics, including reaction dynamics (IUPAC Recommendations 1996)' on page 165 (https://doi.org/10.1351/pac199668010149)