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Viscosity is the "thickness" or "thinness" of a fluid; it´s a property of fluids describing their internal resistance to flow and may be thought of as a measure of fluid friction. Rheology is the field of science that deals with viscosity; viscosity is measured with a viscometer.

If the viscosity of a fluid is constant (neglecting temperature and pressure effects) it is said to be a Newtonian fluid. Non-Newtonian fluids exhibit a variation of viscosity depending on gradients within the flow field, the history that a fluid 'particle' experiences on its flow path, etc. If the viscosity of a fluid depends solely on the gradients within the flow field it is called generalized Newtonian or purely Newtonian.

Generally, viscosity is measured at 25ºC (standard state).

The viscosity of fluids is either given as absolute or dynamic viscosity η (Pa·s = Ns/m = kg/ms) or as kinematic viscosity ν (m/s). Both terms are related via the fluid density ρ to each other: ${\displaystyle \eta =\nu \cdot \rho }$. The old smaller cgs physical unit for dynamic viscosity is poise after Jean Louis Marie Poiseuille (1797-1869): 1 poise = 100 centipoise = 1 g/cms = 0.1 Pa·s. The old unit for kinematic viscosity is stokes (in U.S. called stoke) after George Gabriel Stokes (1819-1903): 1 stokes = 1 cm/s = 0.0001 m/s.

Methanol is "thin", having a low viscosity, while vegetable oil is "thick" having a high viscosity.

Some dynamic viscosities of Newtonian fluids are listed below:

Gases (at 0 °C):

hydrogen 8.4 × 10 Pa·s

air 17.4 × 10 Pa·s

xenon 21.2 × 10 Pa·s

Liquids (at 20 °C):

ethyl alcohol 0.248 × 10 Pa·s

acetone 0.326 × 10 Pa·s

methanol 0.59 × 10 Pa·s

benzene 0.64 × 10 Pa·s

water 1.025 × 10 Pa·s

nitrobenzol 2.0 × 10 Pa·s

mercury 17.0 × 10 Pa·s

sulfuric acid 30 × 10 Pa·s

olive oil 81 × 10 Pa·s

castor oil 985 × 10 Pa·s

glycerol 1485 × 10 Pa·s

pitch 10 Pa·s

Contrary to many assertions, glass is an amorphous solid, not a liquid, and it does not flow, but still we can talk about its viscosity. See the article on glass for more details on this.

Many fluids such as honey have a wide range of viscosity.

Viscosity is also an out-of-print image and animation editing utility published by Sonic Foundry. It can work with PNG, GIF, JPG/JPEG, BMP, AVI and its native VSC format.