Viscosity is a measure of the resistance of a fluid which is being deformed by either shear stress or extensional stress. In general terms it is the resistance of a liquid to flow, or its "thickness". Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction. [Wikipedia]. Please note that the general concept of Viscosity is dimensionally ambiguous and thus is not associated with a DimensionVector. Narrower definitions including DynamicViscosity and KinematicViscosity are preferable.