Bulk Modulus is a measure of the resistance of a fluid to compression. It is defined as the ratio of pressure stress to volumetric strain.
The value of bulk modulus of a fluid is required to determine Mach number. Mach number (M or Ma) is a dimensionless quantity. It represents the flow of velocity beyond the boundary when compared to the local speed of sound. It is used to determine the resistance of fluid or gas in its compressed state.