Friction in fluids is called viscosity. Viscosity exists in both liquids and gases, and is essentially a frictional force between adjacent layers of fluid as the layers move past one another. Viscosity is “thickness” or “internal friction”. Thus, water is “thin”, having a lower viscosity, while honey is “thick”, having a higher viscosity.

When water is flowing through a pipe, the layer of water touching the wall is at rest, the water in the center is moving fastest. The viscosity of different fluids can be expressed quantitatively by a coefficient of viscosity. The stationary layer of fluid retards the flow of the layer just above it, which in turn retards the flow of the next layer, and so on. Thus the velocity varies continuously from 0 to v, as shown in the figure.