Ans: In many flows of practical interests the viscous forces/effects away from the solid surface are negligible.
The effect of the viscosity near the plate is highest due to no-slip condition but as we move away from the plate (as shown in fig) the inertial and pressure forces dominate.
The region away from solid surface where viscous effects are non-significant are called inviscid flow region and the flow in inviscid flow region is called inviscid flow.
The viscosity of the fluid is NOT zero in this region rather the effects of viscosity are negligible or zero.