Viscosity is an important physical property of fluids that describes how resistant a fluid is to deformation or change in shape. It indicates how strong the internal friction between the liquid molecules is.

A fluid with a high viscosity flows more slowly than a fluid with a lower viscosity. Examples of liquids with high viscosity include honey, while water has low viscosity. Viscosity is usually measured in units such as Pa·s or mPa·s.

In joining technology, viscosity is also used to characterize adhesives. A medium to high viscosity adhesive is easier to use for overhead work than a low viscosity adhesive.