From Glossary of Meteorology
(Also called ideal fluid, perfect fluid.) A nonviscous fluid, that is, a fluid for which all surface forces exerted on the boundaries of each small element of the fluid act normal to these boundaries.
By definition, therefore, the stress tensor reduces to the pressure, a point-function scalar in the fluid. Thus, in the dynamics of an inviscid fluid, as opposed to a real viscous fluid, 1) no restraints are placed on the tangential component of the flow at a solid bounding surface, and 2) there is no dissipation of kinetic into thermal energy within the fluid. In the free atmosphere the flow is often treated as inviscid, and the viscous forces may be neglected for many purposes. Where an inviscid fluid flows along a surface, that surface is said to be a free slip surface.
Sutton, O. G. 1953. Micrometeorology. chap. 2.