Definitions for "Incompressible"
Not compressible; incapable of being reduced by force or pressure into a smaller compass or volume; resisting compression; as, many liquids and solids appear to be almost incompressible.
impossible to compress; resisting compression; "mounds of incompressible garbage"
An incompressible fluid is one whose density is constant everywhere. All fluids behave incompressibly (to within 5%) when their maximum velocities are below Mach 0.3.