The Collaborative International Dictionary of English v.0.48:

Viscosity \Vis*cos"i*ty\, n. [Cf. F. viscosit['e], LL.
   viscositas.]
   1. The quality or state of being viscous.
      [1913 Webster]

   2. (Physics) A property possessed by a viscous fluid, being a
      resistance to the forces causing a fluid to flow, caused
      by interactions between the molecules of the fluid and
      between the fluid and the walls of the conduit through
      which it moves; also, a measure of such a property.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

viscosity
    n 1: resistance of a liquid to shear forces (and hence to flow)
         [syn: viscosity, viscousness]