The simultaneous precipitation of a normally soluble component with a macro-component from the same solution by the formation of mixed crystals, by adsorption, occlusion or mechanical entrapment.
Source:
Orange Book, 2nd ed., p. 86 (https://media.iupac.org/publications/analytical_compendium/)
See also:
PAC, 1979, 51, 1195. 'Separation and Preconcentration of Trace Substances. I-Preconcentration for Inorganic Trace Analysis' on page 1199 (https://doi.org/10.1351/pac197951051195)