branching index

in polymers
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, s b 2, to that of an otherwise identical linear molecule, s l 2, with the same relative molecular mass in the same solvent and at the same temperature, i.e. g = s b 2 s l 2.
Purple Book, p. 51