The absolute or dynamic viscosity (η) in centipoise (cP) is a measure of the internal resistance of a fluid to flow. Absolute viscosity is typically measured with a rotational viscometer. Lubricants and base oils are usually described in terms of their kinematic viscosity (ν) in centistokes (cSt), which is defined as the ratio of the kinematic viscosity to the density of a fluid. Kinematic viscosity is typically measured with a flow-type viscometer; an operator measures the length of time that it takes for a fluid to flow through a capillary tube. Thus, kinematic viscosity corresponds to the speed at which a fluid flows under the application of a certain force, typically gravity. The terms “heavier” and “lighter” are used casually to refer to oils and lubricants that have higher and lower viscosities, respectively.