Mach number

If an object travels through a 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 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 object traveling at Mach 1 will depend on its altitude and the atmospheric conditions. High speed flight can be classified in six categories:

Subsonic            M < 1
Sonic               M = 1
Supersonic          M > 1
Transsonic     0.8 < M < 1.3
Hypersonic      5 < M < 10
Hypervelocity       M > 10 and above

An objects that moves at less than Mach 1 sends out sound waves in front of itself. Upon reaching Mach 1, it meets its own sound (the sound barrier). Above Mach 1, the plane will penetrate this barrier, when it does so it is said to break the sound barrier. Upon breaking the sound barrier, a loud booming noise, known as a sonic boom can be heard.

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