Fluid flow which is not subjected to influence of the fluid viscosity.
In this case Navier-Stokes fluid flow momentum equation simplifies to Eauler fluid flow momentum equation.
This flow regime happens for high Reynolds numbers {\rm Re} \gg 1, which originates from:
The Reynolds numbers should not necessarily to be very high to induce a inviscid flow and many laimnar flows {\rm Re} \ll 2,000 behave as inviscid
In many practical applications the fluid viscosity can not be neglected even for low-viscosity fluids.
This happens for example, when fluid flow is happening close to solid boundaries which interaction (a friction) with the fluid depends substantially on fluid viscosity.
These effects can be accounted in inviscid flow model through the impact of dissipative contact forces.
See also
Physics / Mechanics / Continuum mechanics / Fluid Mechanics / Fluid Dynamics / Fluid flow