Viscosity is a property of a liquid due to the collisions between neighboring particles in a fluid that are moving at different velocities in different layers. When the fluid is forced through a tube, the particles of the fluid generally move more quickly near the tube axis and more slowly near its walls. 

Viscosity expresses the magnitude of internal friction in a fluid, as measured by the force per unit area resisting uniform flow.

Example, oil is more viscous than water.