Mach number

If an object travels through some medium, then its Mach number is the ratio of the object's speed to the speed of sound in that medium. It is a dimensionless number, typically used to describe the speed of fast aircraft. Mach 1 is equal to the speed of sound, Mach 2 is twice that speed, etc. Since the speed of sound increases as the temperature increases, the actual speed of an aircraft traveling at Mach 1 will depend on its altitude and the atmospheric conditions. High speed flight can be classified in six general categories

Subsonic            M < 1
Sonic               M = 1
Supersonic          M > 1
Transsonic     .8 < M < 1.3
Hypersonic      5 < M < 10
Hypervelocity       M > 10 and above

Aerodynamic scientists love the Mach number because that one number generates much information like the: temperature, pressure, power, density of air.

The Mach number is named after Austrian physicist and philosopher Ernst Mach.