Definition
The Macroscopic Viscosity increase caused due to enhanced mixing in a Turbulent Flow is called Eddy Viscosity.
Explanation
When a flow turns turbulent. The diffusion in that flow is greatly enhanced due to the increased convection induced by the Turbulence. This is known as enhanced mixing.
If Viscosity is the internal resistance to flow, then the enhanced mixing causes increased Viscosity in those regions. This resistance is called Eddy Viscosity.
Although he viscosity of the fluid is the same at the molecular level, it is much higher on a Macroscopic level.