Compressibility may be defined as the ratio of change in volume of the fluid to the original volume . It can be mathematically expressed as
(change in volume/original volume)*100.
It is often represented in percentage that is why we are multiplying it with 100.
The fluids in which there is change in volume, density and specific weight occurs due to application of pressure are called as compressible fluids.
The fluids in which there is no change in volume, density and specific weight occurs due to change in pressure are known as incompressible fluids.