An ideal fluid is one that has the following properties :
(1) It is incompressible, i.e., its density has a constant value throughout the fluid.
(2) Its flow is irrotational, ie., the flow is steady or laminar. In an irrotational flow, the fluid doesn't rotate like in a whirlpool and the velocity of the moving fluid at a specific point doesn't change over time. (Many fluids change from laminar to turbulent flow as the speed of the fluid increases above some specific value. This can dramatically change the properties of the fluid.)
(3) Its flow is nonviscous or inviscid, i.e., internal friction or viscosity is zero so that no energy lost due the motion of the fluid.
fluids doesn't have fixed shape so can be deformed.
fluids offers no permanent resistance to shearing so they have elastics