/ mɑːk; NAmE ; mæk/ noun
[ U ] (often followed by a number) a measurement of speed, used especially for aircraft. Mach 1 is the speed of sound :
a fighter plane with a top speed of Mach 3 (= 3 times the speed of sound)
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WORD ORIGIN
1930s: named after E. Mach (1838-1916), Austrian physicist and philosopher of science.