An ideal fluid is one that is frictionless and incompressible. It has zero viscosity and it cannot sustain a shear stress at any point.

An understanding of two-dimensional and three-dimensional flow of ideal fluid provides the engineer with a much broader approach to real-fluid flow situations. Although no ideal fluid actually exists, many real fluids have small viscosity and the effects of compressibility may be small. Applications include the motion of a solid through an ideal fluid are applicable with slight modification to the motion of an aircraft through the air, of a submarine through the oceans, flow through the passages of a pump or compressor, or over the crest of a dam.