Scattering of radiation in a generally backward direction. In the assay of radioactivity, it applies to the scattering of radiation into the radiation detector from any material except the sample and the detector. In light scattering, it is said to occur when the scattering angle, θ, is 180°. Conversely, forward scattering occurs when θ is 0°.
Sources:
Orange Book, 2nd ed., p. 213 (https://media.iupac.org/publications/analytical_compendium/)
PAC, 1983, 55, 931. 'Definitions, terminology and symbols in colloid and surface chemistry. Part 1.14: Light scattering (Provisional)' on page 932 (https://doi.org/10.1351/pac198355060931)