Fluid

Substance which continuously deforms under an applied shear stress, including liquids and gases

Depending on the relationship between shear stress and the rate of strain and its derivatives, fluids can be characterized as one of the following:

Newtonian fluids follow Newton's law of viscosity and may be called viscous fluids.

Newtonian and incompressible fluids do not actually exist, but are assumed to be for theoretical settlement. Virtual fluids that completely ignore the effects of viscosity and compressibility are called perfect fluids.