**Viscosity** is the "thickness" or "thinness" of a fluid; it is 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 **25°C** (standard state).

The viscosity of fluids is either given as absolute or **dynamic viscosity** *η* (1 Pa·s = 1 N·s/m^{2} = 1 kg/m·s) or as **kinematic viscosity** *ν* (m^{2}/s). Both terms are related via the fluid density *ρ* to each other: . The old smaller cgs physical unit for dynamic viscosity is *poise* after Jean Louis Marie Poiseuille (1797-1869): 1 poise = 100 centipoise = 1 g/cm·s = 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^{2}/s = 0.0001 m^{2}/s.

It is possible to understand the units of viscosity by considering the force required to shear a fluid. If the viscosity is **v** then a force of **v** newtons per unit area is required to sustain a unit shear rate (shear rate is measured in m/s per m---or just s^{-1}). Then the units of visosity are just Newtons per square meter per s^{-1}. Put another way:
force=viscosity*shear rate*area.

ASTM uses Cps.

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:

Liquids (at 20 °C):- acetone 0.326 × 10
^{-3}Pa·s - benzene 0.64 × 10
^{-3}Pa·s - castor oil 985 × 10
^{-3}Pa·s - ethyl alcohol 0.248 × 10
^{-3}Pa·s - glycerol 1485 × 10
^{-3}Pa·s - methanol 0.59 × 10
^{-3}Pa·s - mercury 17.0 × 10
^{-3}Pa·s - nitrobenzol 2.0 × 10
^{-3}Pa·s - sulfuric acid 30 × 10
^{-3}Pa·s - olive oil 81 × 10
^{-3}Pa·s - pitch 10
^{7}Pa·s - water 1.025 × 10
^{-3}Pa·s

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

**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.