Feature of an extended conjugated
\(\ce{\uppi\ system}\) bearing an electron donor group at one end and an electron acceptor group at the other end.
Source:
PAC, 2022, 94, 353. 'Glossary of terms used in physical organic chemistry (IUPAC Recommendations 2021)' on page 490 (https://doi.org/10.1515/pac-2018-1010)