Mach number is defined as the ratio of velocity of the object to the velocity of sound \(M = \frac{V}{a}\).Mach number is generally associated with flying vehicles. Mach number is a very useful quantity for estimation of compressible flow aerodynamics, particularly high speed aerodynamics,where there is a creation of normal and oblique shock waves. Flying vehicles such as airplanes generally fly in Mach number of 0.8. Supersonic aircrafts like fighter aircrafts fly in Mach number  around of 2.

Re-entry vehicles like space shuttle have Mach number above 25 during its entry to earth. Speed of sound varies with altitude as it is changing with air temperature. So at different altitude on earths atmosphere Mach number is different even if flight speed is same. Also speed of sound is dependent on type of medium. So on mars where there is a different atmosphere than on earth Mach number for the flight vehicle would be different even if it is flying at same flight speed as on earth.\[a = \sqrt {\gamma RT} \]

\[M = \frac{V}{{\sqrt {\gamma RT} }}\]