At a given point in space, the radiant energy incident on a small sphere divided by the cross-sectional area of that sphere. It is used in photochemistry to specify the energy delivered in a given time interval (for instance by a laser pulse).
See also: dose, photon fluence
Green Book, 2nd ed., p. 31 (https://dev.goldbook.iupac.org/files/pdf/green_book_2ed.pdf)
ISO 31-05: 1992 (http://www.iso.org/iso/catalogue_detail.htm?csnumber=3635)