The Mach Number is a dimensionless unit used to represent the speed of an object moving through a fluid, typically 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 greater than 1 signifies supersonic speeds. The speed of sound varies with factors such as temperature and pressure, which means the Mach Number can change depending on the environment. In aviation, understanding Mach Numbers is crucial for aircraft design and performance, especially for high-speed jets and spacecraft, where different regimes like subsonic, transonic, and supersonic flight are important considerations.