Source : Webster's Revised Unabridged Dictionary (1913)

Viscosity \Vis*cos"i*ty\, n. [Cf. F. viscosit['e], LL.
   viscositas.]
   1. The quality or state of being viscous.

   2. (Physics) A quality analogous to that of a viscous fluid,
      supposed to be caused by internal friction, especially in
      the case of gases.

Source : WordNet®

viscosity
     n : resistance of a liquid to sheer forces (and hence to flow)
         [syn: {viscousness}]