The viscosity of a fluid is a measure of its resistance to shear stress. The reciprocal of viscosity is fluidity which is a measure for the flowability (fluidity) of a fluid. The greater the viscosity, the thicker (less fluid) the fluid. The lower the viscosity, the thinner (more fluid) it is. A lower viscosity means that the fluid can flow faster under the same conditions. The term viscosity usually refers to shear. However, it can also be measured in strain. As the particles of viscous liquids are more strongly bonded to one another and thus are less mobile, this is referred to as internal friction. This is not solely due to the attraction forces between the particles of the fluid (cohesion). Since the viscosity of solids is generally very high and therefore difficult to determine, alternative terms such as ductility, brittleness or plasticity are used in this case. Viscosity plays a central role in rheology (flow studies) and is important in practically all areas in which fluids are used or occur (e.g. in paints, varnishes, lubricants, plastics and adhesives, in foodstuffs such as doughs and puddings, but also in medicine).