https://doi.org/10.1351/goldbook.C01081
A substitution reaction (generally aromatic) in which the entering group takes up a position adjacent to that occupied by the leaving group. For example,
Source:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1097 [Terms] [Paper]