A part of a molecular entity (or atom or group of atoms) in which electronic excitation associated with a given emission band is approximately localized. (Analogous to chromophore for absorption spectra).
Source:
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2253 (https://doi.org/10.1351/pac199668122223)