An imaginary fluid devoid of viscosity and thermal conductivity. There is no internal friction in an ideal fluid, that is, there are no shear stresses between two neighbouring layers; it is continuous and has no structure. Such an idealisation is admissible for many of the flows considered in fluid mechanics, and it gives a good description of real flows of liquids and gases far enough from the solid surfaces washed by the flow and from interfaces with a stationary medium. Models of an ideal fluid make it possible to find theoretical solutions to the problems of the motion of liquids and gases in channels of various shapes, of the intersection of jets and of the flow past bodies.