The total energy emitted, transferred or received as radiation in a defined period of time (Q = ∫ Q λ d λ). It is the product of radiant power, P, and time, t: Q = P t when the radiant power is constant over the time considered.
Sources:
Green Book, 2nd ed., p. 30 (https://goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
ISO 31-05: 1992 (http://www.iso.org/iso/catalogue_detail.htm?csnumber=3635)
PAC, 1996, 68, 2223. 'Glossary of terms used in photochemistry (IUPAC Recommendations 1996)' on page 2268 (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 988 (https://doi.org/10.1351/pac199668040957)