https://doi.org/10.1351/goldbook.M03878
A bimolecular process formally involving the exchange of a bond (or bonds) between similar interacting chemical species so that the bonding affiliations in the products are identical (or closely similar) to those in the reactants. For example: 
(The term has its origin in inorganic chemistry with a different meaning, but this older usage is not applicable in physical organic chemistry.)