Same as radiant exitance, M. It is the integration of the radiant power, P, leaving a source over the solid angle and over the whole wavelength range.
Source:
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2276 (https://doi.org/10.1351/pac199668122223)