The document discusses compressibility and the bulk modulus of fluids. It defines compressibility as the property of fluids to change in volume with external pressure. Compressibility decreases with increasing pressure as the bulk modulus, defined as the ratio of compressive stress to volumetric strain, increases. The bulk modulus is a measure of a fluid's elasticity and compressibility is its reciprocal. The bulk modulus is not constant and increases with pressure for fluids. Temperature also affects the bulk modulus for liquids, with a decrease as temperature increases. Examples are given for water and air at normal temperature and pressure.