A hypothetical sphere, impenetrable to the surrounding medium, displaying in a hydrodynamic field the same frictional effect as an actual polymer molecule. The size of a hydrodynamically equivalent sphere may be different for different types of motion of the macromolecule, e.g. for diffusion and for viscous flow.
Source:
Purple Book, 1st ed., p. 60 (http://old.iupac.org/publications/books/author/metanomski.html)