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).
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1129 (https://doi.org/10.1351/pac199466051077)