For instance, in a fluid such as water the stresses which arise from shearing the fluid do not depend on the distance the fluid has been sheared rather, they depend on how quickly the shearing occurs. In other materials, stresses are present which can be attributed to the rate of change of the deformation over time. Stresses which can be attributed to the deformation of a material from some rest state are called elastic stresses. For instance, if the material were a simple spring, the answer would be given by Hooke's law, which says that the force experienced by a spring is proportional to the distance displaced from equilibrium. In materials science and engineering, one is often interested in understanding the forces or stresses involved in the deformation of a material.

In a general parallel flow, the shear stress is proportional to the gradient of the velocity. A fluid that has zero viscosity is called ideal or inviscid. Zero viscosity (no resistance to shear stress) is observed only at very low temperatures in superfluids otherwise, the second law of thermodynamics requires all fluids to have positive viscosity. For example, the viscosity of a Newtonian fluid does not vary significantly with the rate of deformation. However, the dependence on some of these properties is negligible in certain cases. In general, viscosity depends on a fluid's state, such as its temperature, pressure, and rate of deformation. For a tube with a constant rate of flow, the strength of the compensating force is proportional to the fluid's viscosity. This is because a force is required to overcome the friction between the layers of the fluid which are in relative motion. Experiments show that some stress (such as a pressure difference between the two ends of the tube) is needed to sustain the flow. For instance, when a viscous fluid is forced through a tube, it flows more quickly near the tube's axis than near its walls. Viscosity quantifies the internal frictional force between adjacent layers of fluid that are in relative motion. For liquids, it corresponds to the informal concept of 'thickness': for example, syrup has a higher viscosity than water. The conversion of conventional measures into cSt is inaccurate, particularly within the 1 to 9.5 cSt range (see DIN 1342, viscosity of NEWTONian liquids).The viscosity of a fluid is a measure of its resistance to deformation at a given rate. If values higher than 1000 cSt are required, the data given from 60 cSt can be multiplied accordingly by 10, 100, 1000 etc. The table can also alternatively be used to assign corresponding values to one another. The former units are no longer approved and can be converted to m²/s using Fig. To ensure correct usage, the Stokes units and the units still currently in general use must be converted into the SI unit m 2/s. These are: degrees Engler (✮) in Germany, Saybolt seconds (S˝) in USA and Redwood seconds (R˝) in Britain. The conventional units of viscosity are unsuitable as a basis for calculation, but are still used in commerce. The Stokes unit ceased to be officially valid in 1978. The hundredth part thereof is a Centi stokes (cSt). The unit used here is m 2/s and is called – in honour of the Irish physicist STOKES – a Stokes (St). However, most measurements are still made in centistokes, e.g. Thus water at 20 ✬ has a kinematic viscosity ν = 1,002 Kinematic viscosity (ν) designates the quotient of the dynamic viscosity of the fluid handled and its density.