A backward dragging force which oppose the relative motion between two successive layers of liquid

Internal resistance offered by liquid molecules..

Viscosity is the resistance of a fluid to flow.For example, oil has a high viscosity, but when we put it in the car and heat it up, it becomes less viscous and thus flows smoothly through an engine and other parts of the car. Higher the viscosity, slow will be the liquid flow. Viscosity decreases with increase in temperature, with some exceptions.Viscosity is sometimes also understood as frictional forces that act in between fluid and the surface of contact. The surface can either be a solid surface like a pipe and water can be the fluid. Now the resistance provided by the pipe to the water flow is viscosity. However, viscosity can also arise in between two fluids flowing at different velocities.

Viscosity is resistance to flow.
The\xa0viscosity\xa0of a fluid is a measure of its resistance to gradual deformation by shear stress or tensile stress. For liquids, it corresponds to the informal concept of “thickness”; for example, honey has a much higher\xa0viscosity\xa0than water.

Viscocity is the property of fluid by virtue of which an external fractional force comes in play when the fluid is in motion in the form of layers having relative motion.