Ratio of radiant exitance of a thermal radiator to that of a full radiator (black body) at the same temperature. In ISO this quantity is called emissivity.
Sources:
Green Book, 2nd ed., p. 31 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
ISO 31-05: 1992 (https://www.iso.org/iso/catalogue_detail.htm?csnumber=3635)
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2239 (https://doi.org/10.1351/pac199668122223)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 972 (https://doi.org/10.1351/pac199668040957)