The bulk modulus ( or ) of a substance measures the substance's
resistance to uniform compression. It is defined as the ratio of
the infinitesimal pressure increase to the resulting relative
decrease of the volume. Its base unit is the pascal. The bulk
modulus can be formally defined by the equation where is
pressure, is volume, and denotes the derivative of pressure with
respect to volume. Equivalently Other moduli describe the
material's response (strain) to other kinds of stress: the shear
modulus describes the response to shear, and Young's modulus
describes the response to linear stress. For a fluid, only the
bulk modulus is meaningful. For an anisotropic solid such as
wood or paper, these three moduli do not contain enough
information to describe its behaviour, and one must use the full
generalized Hooke's law. Strictly speaking, the bulk modulus is
a thermodynamic quantity, and in order to specify a bulk modulus
it is necessary to specify how the temperature varies during
compression: constant-temperat