A mechanism intermediate between two extremes, for example a nucleophilic substitution intermediate between S N 1 and S N 2, or intermediate between electron transfer and S N 2.
Source:
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1090 (https://doi.org/10.1351/pac199466051077)