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.
ISO 31-5: 1992 (Quantities and Units - Part 5: Electricity and Magnetism.)
PAC, 1996, 68, 2223 (Glossary of terms used in photochemistry (IUPAC Recommendations 1996)) on page 2239
PAC, 1996, 68, 957 (Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)) on page 972