The resistance of a liquid to flowing is called

Viscosity

Viscosity is the property of the liquid to resist it's flow. It opposes the relative motion of molecules of the liquid over a surface. It is resistance to the gradual deformation of shear stress or tensile stress of the liquid. It imparts the concept of thickness. Higher the viscosity more thicker will be the liquid and lesser will be it's flow, where lower the viscosity, less thicker liquid more will be it's flow.