The difference between the actual π-electron energy of a molecular entity and the π-electron energy of a hypothetical species with a localized form of the π-electron system. These energies are normally evaluated within Hückel molecular orbital theory.
Source:
PAC, 1999, 71, 1919. 'Glossary of terms used in theoretical organic chemistry' on page 1933 (https://doi.org/10.1351/pac199971101919)