The radiant exitance, M, at wavelength λ per unit wavelength interval. The SI unit is W m-3, but a commonly used unit is W m-2 nm-1.
Source:
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2276 (https://doi.org/10.1351/pac199668122223)