A tensor correlating the frictional force, F, opposing the motion of a particle in a viscous fluid and the velocity u of this particle relative to the fluid. In the case of an isolated spherical particle in a viscous isotropic fluid, f is a constant and F = f.u.
Source:
Purple Book, 1st ed., p. 60 (http://old.iupac.org/publications/books/author/metanomski.html)