an ideal fluid
An ideal fluid is a simplified, imaginary fluid that physicists use to make flowing liquids and gases easy to reason about. Real fluids are messy: they stick, swirl, and squash. An ideal fluid strips all that away and keeps only the smooth essentials, the way a frictionless surface simplifies mechanics. It answers a modelling question: what is the simplest fluid whose motion we can describe cleanly?
Precisely, an ideal fluid is assumed to have two key properties. It is incompressible, meaning its density stays constant no matter the pressure, so it never gets squeezed smaller. And it is nonviscous (inviscid), meaning it has no internal friction, so neighbouring layers slide past one another with no drag. We usually also assume the flow is steady, so the pattern of motion at each point does not change with time. With these assumptions the flow becomes tidy enough to describe with two famous results, the continuity equation and Bernoulli's principle.
The ideal fluid is the natural first model for pipes, wings, and rivers, and it captures a great deal of real behaviour surprisingly well. But its power comes from admitting what it leaves out. Real fluids do have viscosity, which is why honey pours slowly and why a real pipe loses pressure to friction along its length; real fluids can compress at high speeds; and real flows often turn turbulent. The ideal fluid is a deliberate idealization, honest as long as we remember it is one.
When we predict how water speeds up as a pipe narrows, or how air pressure drops over a wing, we usually treat the fluid as ideal, no viscosity, no compression, so that A v stays constant and Bernoulli's equation applies cleanly.
The ideal fluid keeps only incompressibility and zero viscosity, making the flow simple enough to solve.
An ideal fluid is an idealization: real fluids have viscosity, can compress, and can go turbulent. It is a useful model, not a real substance.