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.
The term is intended to imply that the correct representation of a structure is intermediate
between two or more Lewis formulae.
Source:
PAC, 1994, 66, 1077
(Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994))
on page 1139