Viscosity was the measurement of the thickness of a liquid. An example of this was blood viscosity.
The Guardians regularly checked the living conditions of the Trill symbionts in the Caves of Mak'ala, including ion concentration, temperature and viscosity. (DS9: "Equilibrium")
In 2370, Mora Pol described Odo in his initial state upon discovery as "a viscous, shapeless form of fluid, a veritable organic broth". (DS9: "The Alternate")
The Silver Blood discovered in 2372 was a metallic compound with high viscosity. (VOY: "Demon")