Viscosity is a measure of the **resistance of a fluid** which is being deformed by either shear stress or tensional stress. In everyday terms (and for fluids only), viscosity is “thickness”. Thus, water is “thin”, having a lower viscosity, while honey is “thick”, having a higher viscosity.

Viscosity describes a fluid’s internal resistance to flow and may be thought of as a measure of fluid friction. See Newton’ expression: the constant **μ** is known as the coefficient of viscosity, the viscosity, the dynamic viscosity, or the Newtonian viscosity.