viscosity

The resistance of a fluid to flow, caused by its internal friction, which makes it resist flowing past a solid surface and makes layers of the fluid resist flowing past other layers. Treacle and other thick, sticky liquids are highly viscous liquids. Water and petrol are runny liquids and have low viscosity.

The term viscosity applies to the motion of an object moving through a fluid as well as the motion of a fluid passing by an object. For the purposes of calculation, many fluids in physics are considered to be perfect, or nonviscous.