The Mach number is a dimensionless unit used to represent the speed of an object moving through a fluid, such as air, compared to the speed of sound in that fluid. It is calculated by dividing the object's speed by the speed of sound at the same conditions. For example, a Mach number of 1 indicates that the object is traveling at the speed of sound, while a Mach number of 2 means it is traveling twice as fast. In aviation and aerospace, the Mach number is crucial for understanding flight dynamics. Different Mach numbers correspond to different flight regimes, such as subsonic (less than 1), transonic (around 1), supersonic (greater than 1), and hypersonic (greater than 5). These regimes affect the design and performance of aircraft and spacecraft.