Compressibility

Compressibility measures how much a substance’s volume changes fractionally when pressure changes. It is the reciprocal of bulk modulus for small changes under the same conditions.

Bulk modulus: Bulk modulus measures resistance to uniform compression as pressure increases. It is the reciprocal of compressibility for small changes under matching conditions.
Pressure: Force exerted per unit area on a surface or within a fluid. Compressibility is defined by how volume responds to a pressure change.
Speed of sound: The speed at which small pressure disturbances propagate through a medium. For a fluid, sound speed depends on density and adiabatic compressibility.
Incompressible fluid: An idealized fluid whose density remains constant despite pressure changes. It is a simplifying limit that neglects the finite compressibility of real liquids.
Equation of state: An equation relating a substance’s pressure, volume, temperature, and sometimes other state variables. Its pressure–volume slope determines compressibility at a specified thermodynamic state.
Volume: The three-dimensional space occupied by matter or a region. Its fractional change, rather than its absolute change, defines compressibility.
Acoustic impedance: The ratio of sound pressure to particle velocity in a propagating acoustic wave. Compressibility helps set the acoustic properties that determine impedance.
Elasticity: The ability of a material to deform under stress and recover when the stress is removed. Compressibility is the volumetric part of elastic response, unlike shape-changing shear response.
Isothermal compressibility: Fractional volume change per unit pressure change when temperature is held constant. It specifies the thermal constraint used when measuring compressibility.
Strain: A dimensionless measure of deformation relative to an object's original size. Volumetric strain is the fractional volume change used in the definition.

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