The cross-section per unit volume of a given material for a specified process. For a pure nuclide, it is the product of the microscopic cross-section and the number of target nuclei per unit volume; for a mixture of nuclides, it is the sum of such products.
Sources:
Orange Book, 2nd ed., p. 218 (https://media.iupac.org/publications/analytical_compendium/)
PAC, 1994, 66, 2513. 'Nomenclature for radioanalytical chemistry (IUPAC Recommendations 1994)' on page 2517 (https://doi.org/10.1351/pac199466122513)