The bulk modulus of a medium is the property of the medium that determines the fractional change in a volume of the medium when the pressure on it changes.

A higher bulk modulus indicates a less compressible medium.  For example, steel has a bulk modulus of 160 x 10⁹ Pa while air has a bulk modulus of about 10⁵ Pa.

The bulk modulus is always positive. Increasing the pressure (Dp > 0) causes a decrease in the volume (DV < 0) and decreasing the pressure (Dp < 0) causes an increase in the volume (DV > 0).

The SI unit of bulk modulus is the same as that of pressure N/m² or Pascal (Pa).