A coefficient relating the average shear stress within a turbulent flow of water or air to the vertical gradient of velocity. The eddy viscosity depends on the fluid density and distance from the river bed or ground surface. The concept of eddy viscosity is fundamental to the von Karman–Prandtl description of the velocity profile in turbulent flow, and is important in determining rates of evaporation or cooling by wind, and the shear stress exerted by rivers on moving particles on their beds.