The rejection of a fraction of macromolecules, or of impurities, or both, from growing crystals. The rejected macromolecules are usually those of insufficient relative molecular mass, or differing in constitution or configuration (e.g. branching, tacticity, etc.).
Source:
Purple Book, 1st ed., p. 85 (http://old.iupac.org/publications/books/author/metanomski.html)