A quantity expressing the magnitude of internal friction, as measured by the force per unit area resisting a flow in which parallel layers unit distance apart have unit speed relative to one another.

Viscosity

Explanation:

Viscosity can be defined as a quantity expressing the magnitude of internal friction, as measured by the force per unit area resisting a flow in which parallel layers unit distance apart have unit speed relative to one another.

It is basically the opposite of fluidity, a process of ease of flow.

Mathematically, Viscosity = Shear stress/Shear rate

The S. I unit for Viscosity is Pascal-second.