An inviscid fluid which one which does not exhibit viscosity, i.e., the fluid has no internal resistance to flow. This also means that the flow has no friction or shear stresses. No real world fluid flow has this property, however, many flows have a very small viscosity so as to be negligible and the simplifying assumption of an inviscid flow can be made.
An inviscid flow is irrotational; it is the effect of viscosity that causes rotation in a fluid field. Consequently, we can conclude that as time passes, a vortex in the flow is preserved.
Inviscid flows are notable for demonstrating d'Alembert's Paradox that a body in an inviscid, incompressible fluid has no drag force exerted on it by the fluid (no profile drag). It turns out that the phenomenon of profile drag is caused by viscous forces between the body and the fluid, and within the fluid, resulting in the presence of a boundary layer which has a lower velocity and loses momentum to the body in the direction of the flow.