For mono-directional radiation, the energy traversing in a time interval over a small area perpendicular to the direction of the energy flow, divided by that time interval and by that area.
See also: intensity
Source:
PAC, 1994, 66, 2513. 'Nomenclature for radioanalytical chemistry (IUPAC Recommendations 1994)' on page 2519 (https://doi.org/10.1351/pac199466122513)