In Physics, a fluid is a substance that continually deforms (flows and cannot resist deformation) under applied shear stress or external force. Fluids include gases, liquids, and plasma.

All fluids are compressible (that is, their density increases under increasing pressure) to some extent, but liquids are much less compressible than gases and are generally considered incompressible. Even gases may be treated as incompressible provided the airflow speeds involved are not high. For subsonic airflow over an airplane below about 150 m/s (492 ft./s or about 336 mph), air may be treated as incompressible, i.e., the density remains the same throughout the flow. At higher speeds, the effects of compressibility must be taken into account.

Related keywords

  • Superfluid
  • Superfluidity