The bulk modulus B gives the change in volume of a solid substance as the pressure on it is changed,
The compressibility K or , is given by
The isothermal compressibility, is given by
(Note: in Hansen and McDonald the isothermal compressibility is written as ). where is the temperature, is the particle number density given by
where is the total number of particles in the system, i.e.
The adiabatic compressibility, is given by
where is the entropy.