A model simulating the hydrodynamic properties of a chain macromolecule consisting of a sequence of beads, each of which offers hydrodynamic resistance to the flow of the surrounding medium and is connected to the next bead by a rigid rod which does not. The mutual orientation of the rods is random.
Source:
Purple Book, 1st ed., p. 60 (http://old.iupac.org/publications/books/author/metanomski.html)