In fluid dynamics, inviscid flow is the flow of an inviscid fluid which is a fluid with zero viscosity.
The Reynolds number of inviscid flow approaches infinity as the viscosity approaches zero.  When viscous forces are neglected, such as the case of inviscid flow, the Navier–Stokes equation can be simplified to a form known as the Euler equation. This simplified equation is applicable to inviscid flow as well as flow with low viscosity and a Reynolds number much greater than one.  Using the Euler equation, many fluid dynamics problems involving low viscosity are easily solved, however, the assumed negligible viscosity is no longer valid in the region of fluid near a solid boundary (the boundary layer) or, more generally in regions with large velocity gradients which are evidently accompanied by viscous forces.
The flow of a superfluid is inviscid.
Inviscid flows are broadly classified into potential flows (or, irrotational flows) and rotational inviscid flows.