Viscosity is a measure of the resistance of a fluid to being deformed either shear stress or extensional stress.

It is commonly perceived as "thickness", or resistance to flow.

Viscosity describes a fluids internal resistance to flow and may be thought of as a measure of fluid friction. Thus, water is "thin", having a lower viscosity, while vegetable oil is "thick" having a higher viscosity. All real fluids (except superfluids) have some resistance to stress, but a fluid which has no resistance to shear stress is known as an ideal fluid or inviscid fluid.

The study of viscosity is known as rheology.

The word derives from the Latin word "viscum" for mistletoe.

A  glue was made from mistletoes berries and used for lime twigs to catch birds

Viscosity coefficients

When looking at a value of viscosity, the number that one most often sees is the coefficient of viscosity.

There are several different viscosity coefficients depending on the nature of applied stress and nature of the fluid. They are introduced in the main books on hydrodynamics and rheology.

  • Dynamic viscosity determines the dynamics of an incompressible Newtonian fluid;
  • Kinematic viscosity is the dynamic viscosity divided by the density for a Newtonian fluid;
  • Volume viscosity determines the dynamics of a compressible Newtonian fluid;
  • Bulk viscosity is the same as volume viscosity
  • Shear viscosity is the viscosity coefficient when the applied stress is a shear stress (valid for non-Newtonian fluids);
  • Extensional viscosity is the viscosity coefficient when the applied stress is an extensional stress (valid for non-Newtonian fluids).

Shear viscosity and dynamic viscosity are much better known than the others.

That is why they are often referred to as simply viscosity. Simply put, this quantity is the ratio between the pressure exerted on the surface of a fluid, in the lateral or horizontal direction, to the change in velocity of the fluid as you move down in the fluid (this is what is referred to as a velocity gradient).

For example, at "room temperature", water has a nominal viscosity of 1.0 ×  10-3 Pa∙s and motor oil has a nominal apparent viscosity of 250 ×  10-3 Pa∙s.

Extensional viscosity is widely used for characterizing polymers.
Volume viscosity is essential for Acoustics in fluids, see Stokes law (sound attenuation)