The process of random incorporation of comparatively small quantities of other phases (e.g. water, dust, particles, etc.) in the bulk of a precipitate during its formation.
The deliberate capture of small quantities of such phases by the deliberate addition of solids to a liquid phase. (The term inclusion is not recommended).
Source: Orange Book, 2nd ed., p. 85 (https://media.iupac.org/publications/analytical_compendium/)