Viscosity is a measure of the resistance of a fluid which is being deformed by either shear stress or extensional stress. It is commonly perceived as "thickness", or resistance to flow. Viscosity describes a fluid's 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. For example a high viscosity magma will create a tall volcano, because it cannot spread fast enough, low viscosity lava will create a shield volcano, which is large and wide.^[1] The study of viscosity is known as rheology.

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