The viscosity of a fluid is a measure of its resistance to gradual deformation by shear stress or tensile stress. For liquids, it corresponds to the informal concept of “thickness”. For example, honey has a much higher viscosity than water.

In other words, Viscosity is a property arising from collisions between neighboring particles in a fluid that are moving at different velocities. When the fluid is forced through a tube, the particles which comprise the fluid generally move more quickly near the tube’s axis and more slowly near its walls: therefore some stress, (such as a pressure difference between the two ends of the tube), is needed to overcome the friction between particle layers and keep the fluid moving. For the same velocity pattern, the stress required is proportional to the fluid’s viscosity. In common parlance, however, a liquid is said to be viscous if its viscosity is substantially greater than water’s, and may be described as mobile if the viscosity is noticeably less than water’s. If the viscosity is very high, for instance in pitch, the fluid will appear to be a solid in the short term.