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In physics, a fluid is considered incompressible if its density remains constant regardless of pressure changes. This means that as pressure is applied to an incompressible fluid, its volume does not decrease. In reality, no fluid is perfectly incompressible; however, many fluids, such as liquids and gases at low speeds, can be approximated as incompressible for practical purposes. This approximation simplifies many fluid dynamics calculations.

The concept of incompressibility is particularly useful in aerodynamics and hydrodynamics. For instance, when analyzing the flow of water in pipes or the airflow around an aircraft at low speeds, assuming incompressibility significantly reduces the complexity of the governing equations. The Mach number is a key parameter used to determine if a fluid can be treated as incompressible. Generally, if the Mach number is below 0.3, the fluid's compressibility effects are considered negligible, and it can be treated as incompressible.

While liquids are generally much closer to being incompressible than gases, even they exhibit some degree of compressibility. For example, water's density changes slightly under extreme pressures, such as those found deep in the ocean. However, for most engineering applications involving liquids, the assumption of incompressibility is valid and widely used. Gases, on the other hand, are highly compressible due to the large spaces between their molecules. Their density changes significantly with pressure and temperature.