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]

Moby Thesaurus II by Grady Ward, 1.0:

71 Moby Thesaurus words for "viscosity":
   adhesiveness, bullheadedness, clabbering, clamminess, closeness,
   clotting, coagulation, cohesiveness, colloidality, compactness,
   congestedness, congestion, consistence, consistency, crowdedness,
   curdling, denseness, density, doughiness, firmness, gelatinity,
   gelatinousness, gluelikeness, gluiness, glutinosity, glutinousness,
   gumlikeness, gumminess, hardness, heaviness, impenetrability,
   impermeability, imporosity, incompressibility, incrassation,
   inspissation, jammedness, jellification, jellylikeness, lentor,
   mucilaginousness, obstinacy, pastiness, persistence, persistency,
   relative density, retention, ropiness, slabbiness, sliminess,
   snugness, solidity, solidness, specific gravity, spissitude,
   stick-to-itiveness, stickiness, stodginess, stringiness,
   stubbornness, syrupiness, tackiness, tenaciousness, tenacity,
   thickening, thickness, tightness, toughness, treacliness,
   viscidity, viscousness