What does the "MACH number" signify?

The MACH number is a dimensionless unit that signifies the ratio of the speed of an aircraft to the speed of sound in the surrounding medium. This is particularly important in aviation and aerospace, as it helps pilots and engineers understand the aircraft's performance relative to the speed of sound, which varies depending on factors like altitude and temperature.

When an aircraft is traveling at a MACH number of 1, it is flying at the same speed as the speed of sound; a MACH number greater than 1 indicates supersonic flight, whereas a MACH number less than 1 indicates subsonic flight. Understanding the MACH number is crucial for safe and efficient flight operations, as it affects aerodynamic characteristics and performance capabilities of the aircraft.