A chemical reaction, the principal product of which is isomeric with the principal reactant. An intramolecular isomerization that involves the breaking or making of bonds is a special case of a molecular rearrangement. Isomerization does not necessarily imply molecular rearrangement (e.g. in the case of the interconversion of conformational isomers).
Source:
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1129 (https://doi.org/10.1351/pac199466051077)