a) In soils engineering, compressibility is a mechanical property that defines the resistance of a water saturated clayey soil to compressional deformation under effective stress. The change in effective stress leads to the flow of porewater, resulting in the change in soil thickness or volume. Usually the compressibility of clay is measured by the one-dimensional (i.e., K0 condition) consolidation testing, and is defined by the compression index.

b) In geophysics, compressibility is defined as the reciprocal of the bulk modulus, which relates the change in volume of a material to the hydrostatic state of stress.
See also effective stress, void ratio, compression index