luminous flux received by a surface divided by the area of that surface.
Sources:
Green Book, 2nd ed., p. 72 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
PAC, 1996, 68, 957. 'Glossary of terms in quantities and units in Clinical Chemistry (IUPAC-IFCC Recommendations 1996)' on page 976 (https://doi.org/10.1351/pac199668040957)
See also:
PAC, 1990, 62, 2167. 'Glossary of atmospheric chemistry terms (Recommendations 1990)' on page 2196 (https://doi.org/10.1351/pac199062112167)