When a fluid is subjected to external forces, it resists flow due to internal friction. Viscosity is a measure of this internal friction.
Kinematic viscosity is a measure of the resistive flow of a fluid under the influence of gravity.
When two fluids of equal volume are placed in identical capillary viscometers and allowed to flow by gravity, a viscous fluid takes longer than a less viscous fluid to flow through the capillary. If one fluid takes 200 seconds to complete its flow and another fluid takes 400 seconds, the second fluid is twice as viscous as the first on a kinematic viscosity scale. These units can be converted to centistokes by following the instructions in ASTM D 2161. The relationship in which viscosity is a constant regardless of shear stress or rate of shear is called Newton's Viscosity Law. Many common solvents, mineral base oils, synthetic base fluids and fully formulated single-grade oils obey Newton's Viscosity Law, and are called Newtonian fluids.