The property of the fluid in which layers between them that are in relative motion suffer an internal frictional force is called the viscosity or viscous drag of a fluid.
It is a characteristic property of a fluid, hence its value varies for different fluids.
Liquids are more viscous than gaseous fluids.
If is the viscosity of a fluid, is the area of each place and is the rate of shear deformation, then we can say that the force exerted on the fluid is,
Shear deformation occurs when shear strain is applied in which the material deforms by sliding past the parallel internal surfaces of the fluid through one another.
The rate of application of shear deformation to a given fluid is .