The ratio of a dimensional characteristic of a macromolecule in a given solvent at a given temperature to the same dimensional characteristic in the θ state at the same temperature. The most frequently used expansion factors are: expansion factor of the mean-square end-to-end distance, αr = sqrt(/o); expansion factor of the radius of gyration αs = sqrt(/o); viscosity expansion factor αη = ([η]/[η]θ)1/3 where [η] and [η]θ are the intrinsic viscosity in a given solvent and in the θ state at the same temperature, respectively. Expansion factors defined by different dimensional characteristics are not exactly equal, nor need they have a constant ratio as a function of relative molecular mass.

Source:
Purple Book, 1^st ed., p. 59 (http://old.iupac.org/publications/books/author/metanomski.html)