The lowering of the particle (or photon) flux density in the neighbourhood of an object due to absorption of particles (or photons) in the object.
Source:
PAC, 1994, 66, 2513. 'Nomenclature for radioanalytical chemistry (IUPAC Recommendations 1994)' on page 2519 (https://doi.org/10.1351/pac199466122513)