Essentially synonymous with resonance. The term is particularly associated with the picture of π-electrons as less localized in an actual molecule than in a Lewis formula (electron dot or Lewis structure). The term is intended to imply that the correct representation of a structure is intermediate between two or more Lewis formulae.
See also: aromatic (Def. 2), delocalization
PAC, 1994, 66, 1077. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)' on page 1139 (https://doi.org/10.1351/pac199466051077)