A parameter, g, characterizing the effect of long-chain branches on the size of a branched macromolecule in solution and defined as the ratio of the mean-square radius of gyration of a branched molecule, < sb2 >, to that of an otherwise identical linear molecule, < sl2 >, with the same relative molecular mass in the same solvent and at the same temperature, i.e. g = /.

Source:
Purple Book, 1^st ed., p. 51 (http://old.iupac.org/publications/books/author/metanomski.html)