Complex sentence: In fluid dynamics, the speed of sound in a fluid medium (gas or liquid) is used as a relative measure of spe speed of an object divided by the speed of sound in the fluid is called the Mach number.
Split into simpler sentences: In fluid dynamics, the speed of sound in a fluid medium (gas or liquid) is used as a relative measure of speed itself. The speed of an object divided by the speed of sound in the fluid is called the Mach number.