a speed relative to the speed of sound Mach number is a ratio of the speed of a body (aircraft) to the speed of sound)
Mach is named after Ernst Mach an Austrian philosopher and physicist.
Mach 1 is about 717mph Mach 2 is twice that. The speed of sound varies due to atmospheric conditions and density of the air.
Most commercial airliners fly below Mach 1
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