The ratio of the rate constants of two reactions that differ only in the identity of the element of the atom in the leaving group, e.g. kBr/kCl. As for isotope effects, a ratio of unity is regarded as a 'null effect'.
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1111 (https://doi.org/10.1351/pac199466051077)