...
Fluid flow which is not subjected to influence of the fluid viscosity.
It may happen for low-viscosity fluids or or for the very high Reynolds numbers.
In this case Navier-Stokes fluid flow momentum equation simplifies to Eauler fluid flow momentum equation.
In many practical applications the fluid viscosity can can not be neglected even for the low-viscous viscosity fluids.
This happens for example, when fluid flow is is restricted by solid boundaries which interaction (a friction) with the fluid depends depends on fluid viscosity.
These effects can be incorporated and can be accounted in in inviscid flow model through the external disspative impact of surface dissipative forces.
See also
...
Physics / Mechanics / Continuum mechanics / Fluid Mechanics / Fluid Dynamics / Fluid flow