Ideal fluid
Also called Nonviscous fluid, Incompressible fluid
An ideal fluid is one that is incompressible and has no viscosity, so its density never changes and no energy is lost to internal friction. AP Physics 1 assumes every fluid is ideal unless a question says otherwise.
Essential knowledge 8.1.A.4 is one sentence long: an ideal fluid is incompressible and has no viscosity. Those are the only two properties the CED names, and each one buys a specific equation.
Incompressible means is fixed no matter what pressure the fluid is under, which EK 8.2.A.3 states separately. That is what lets mass conservation collapse into the continuity equation in terms of volume, .
No viscosity means no internal friction, so no mechanical energy leaks away as thermal energy along the pipe. That is what lets Bernoulli's equation be an equality rather than an inequality.
You are not asked to remember when the assumption applies, because the exam grants it. The AP Physics 1 table of information lists among the exam conventions: "Fluids are assumed to be ideal, and pipes are assumed to be completely filled by fluid, unless otherwise stated." The Unit 8 boundary statement repeats it for the unit.
Worth knowing what the model throws away. A real viscous liquid loses pressure steadily along a level pipe of constant width, where Bernoulli predicts none, and it flows fastest at the centre. The words laminar and turbulent never appear in the AP Physics 1 CED.