Fluid

The fluid is any substance which conforms to the shape of its vessel. It might be either a liquid or a gas.

Compressibility of Fluids

Generally a fluid might be considered incompressible whenever the velocity of the fluid is more than one-third of the speed of sound for the fluid, or when the fluid is a liquid. The treatment of a fluid which is considered incompressible is simple since the density is supposed to be constant, giving an easy relationship for the state of the substance. The disparity of density of the fluid with modifications in pressure is the main factor considered in choosing whether a fluid is incompressible.

Fluids which are compressible have much more complicated equations to deal with, due to density changes, and contain property relationships which differ more quickly than incompressible fluids. Additionally, fixing the state of a liquid can be done simply by knowing its pressure and temperature. Once the substance becomes a gas, the procedure becomes harder.