A real fluid flowing in a pipe experiences frictional forces. There is friction with the walls of the pipe, and there is friction within the fluid itself, converting some of its kinetic energy into heat. The frictional forces that try to prevent different layers of fluid from sliding past each other are called viscous forces. Viscosity is a measure of a fluids resistance to relative motion within the fluid. We can measure the viscosity of a fluid by measuring the viscous drag between two plates.
If you measure the force to keep the upper plate moving with constant velocity v0, you find it is proportional to the area of the plate, and to v0/d, where d is the distance between the plate.
F/A = ηv0/d
The proportional constant η is called the viscosity. The units of η in SI units are Pa-s.
The work done by viscous forces converts ordered energy into thermal energy. For fluid flowing in a long horizontal pipe, the pressure drops along the pipe in the direction of the flow. The faster the fluid is flowing, the larger is the pressure drop.