Mach number (Ma) (pronounced in British English with a Germanic ch, as "mack" or "mark" and as "mock" in American English) is defined as a ratio of the speed of an object or flow relative to the speed of sound in the medium through which it is traveling.
The Mach number is commonly used both with objects traveling at high speed in a fluid, and with high-speed fluid flows inside channels such as nozzles, diffusers or wind tunnels. As it is defined as a ratio of two speeds, it is a dimensionless number. At standard sea level conditions, Mach 1 is 1,225 km/h (761.2 mph) in the atmosphere.
Since the speed of sound increases as the temperature increases, the actual speed of an object traveling at Mach 1 will depend on the fluid temperature around it.
The Mach number is named after Austrian physicist and philosopher Ernst Mach.
A longer and more technical version of the article on Wikipedia