conjugated system (conjugation)

https://doi.org/10.1351/goldbook.C01267
In the original meaning a conjugated system is a molecular entity whose structure may be represented as a system of alternating single and multiple bonds: e.g. CH2=CH–CH=CH2, CH2=CH–C≡N. In such systems, conjugation is the interaction of one p-orbital with another across an intervening σ-bond in such structures. (In appropriate molecular entities d-orbitals may be involved.) The term is also extended to the analogous interaction involving a p-orbital containing an unshared electron pair, e.g. :Cl–CH=CH2.
See also:
delocalization
,
homoconjugation
,
resonance
Source:
PAC, 1994, 66, 1077. (Glossary of terms used in physical organic chemistry (IUPAC Recommendations 1994)) on page 1099 [Terms] [Paper]