What does "MACH number" signify in aviation?

The Mach number signifies the ratio of the speed of the aircraft to the speed of sound in the surrounding medium. In aviation, understanding this concept is crucial as it helps determine the aerodynamic characteristics of the aircraft. At different Mach numbers, the behavior of air around the aircraft changes significantly, impacting lift, drag, and stability. This is particularly important for high-speed aircraft, where compressibility effects become pronounced as they approach and exceed the speed of sound. Thus, the Mach number provides vital information about the aircraft's performance relative to sonic conditions.