Dilatation

Dilatation is described as change in volume (bulk) of the solid without modified of shape. If normal stress components of equal magnitude and similar nature (all tensile or all compressive) are applied on all of the three mutually perpendicular planes of a solid element, the solid shall undergo only volume change without any modification in shape. Such a state of stress (as might be attained in the case of solid subjected to hydrostatic pressure) is called as volumetric stress & the change of volume generated per unit volume of the solid is called as the volumetric strain. The ratio among volumetric stress & volumetric strain is described as Bulk Modulus.

Therefore, Bulk Modulus, K = Volumetric Stress/ Volumetric Strain 

in which     Volumetric strain = Change of Volume/ Original Volume

The state of stress, with equal values of stress components in all the direction is also known as spherical state of stress.