The maximum value of the molar mass or molecular weight of molecules or particles, in a specific polymer solvent system, that can enter into the pores of the porous non-adsorbing material used in size-exclusion chromatography. For particles with molar mass or molecular weight larger than the exclusion limit the separation effect of the size-exclusion chromatography vanishes.
Source:
Purple Book, 1st ed., p. 69 (http://old.iupac.org/publications/books/author/metanomski.html)