The (usually intramolecular) transfer of an atom or group during the course of a molecular rearrangement.
The movement of a bond to a new position, within the same molecular entity is known as 'bond migration'. Allylic rearrangements, e.g.
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exemplify both types of migration.
Source: PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1141 (https://doi.org/10.1351/pac199466051077)