In the study of turbulence in fluids, a common practical strategy for calculation is to ignore the small-scale vortices (or eddies) in the motion and to calculate a large-scale motion with an eddy viscosity that characterises the transport and dissipation of energy in the smaller-scale flow.

Typical values of eddy viscosity used in modelling ocean circulation are in excess of 107 Pa·s.

The transfer of momentum caused by turbulent eddies is often modeled with an effective eddy viscosity  as the momentum transfer caused by molecular diffusion  is modeled with a molecular viscosity.

The effect of turbulent eddies on the flow can be modeled in this was is often refered to as the Boussinesq eddy viscosity assumption and it was first formulated by Boussinesq in 1877.

The eddy viscosity is also commonly called the turbulent viscosity and it is normally written as $mu_t$.