mach number

n. number which shows the speed of an object in relation to the speed of sound

Mach number (Ma) (pronounced: , see IPA) is a dimensionless measure of relative speed. It is defined as the speed of an object relative to a fluid medium, divided by the speed of sound in that medium: where is the Mach number is the velocity of the object relative to the medium and is the velocity of sound in the medium
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Noun
1. the ratio of the speed of a moving body to the speed of sound
(hypernym) ratio

The ratio of true airspeed to the speed of sound. (14CFR1) (FAA4)

The ratio of true airspeed to the speed of sound; e.g., MACH .82, MACH 1.6.
(See AIRSPEED.)