https://doi.org/10.1351/goldbook.T06256
A substitution reaction in which the entering group takes up a position more than one atom away from the atom to which the leaving group was attached:
Source:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1172 [Terms] [Paper]