Bulk Modulus:

This mechanical property for a substance is related to it compressibility, so is also called the modulus of compression. . 
It is constant for a given material.   The higher the value of Bulk Modulus, the more difficult is the material to compress. Compressibility is 1/K.

The characteristic is defined as the negative ratio of change in pressure applied to a block of some material to the fractional change in volume of the block