a number representing the ratio of the speed of an object to the speed of sound in the surrounding medium, as air, through which the object is moving
See Mach number in American Heritage Dictionary 4
also mach number(mäk)
noun Abbr. M
The ratio of the speed of an object to the speed of sound in the surrounding medium. For example, an aircraft moving twice as fast as the speed of sound is said to be traveling at Mach 2.