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Mach number

A unit of speed used to indicate the speed of aircraft, and of spacecraft within the atmosphere. Mach 1 is traveling at the speed of sound; Mach 2, twice the speed of sound, and so on. In dry air at 0°C, the speed of sound is 1,193.3 kilometers per hour (741.5 miles per hour).

Only military aircraft are permitted to reach Mach 1 over the United States. The British-French Concorde airliner did reach supersonic speeds on its transoceanic flights, and had a Mach meter on the forward bulkhead of its passenger cabin.