Mach number

Mach number (Ma or M) (generally pronounced /ˈmɑːk/, sometimes /ˈmɑːx/ or /ˈmæk/) is the speed of an object moving through air, or any fluid substance, divided by the speed of sound as it is in that substance. It is commonly used to represent an object's speed (such as an aircraft or missile), when it is travelling at the speed of sound (or multiples thereof).
An F/A-18 Hornet at transonic speed and displaying the Prandtl–Glauert singularity just before reaching the speed of sound

\ M = \frac {{v_s}}{{u}}

where

\ M is the Mach number
\ v_s is the speed of the source (the object relative to the medium) and
\ u is the speed of sound in the medium

The Mach number is named after Czech/Austrian physicist and philosopher Ernst Mach. Because the Mach number is often viewed as a dimensionless quantity rather than a unit of measure, with Mach, the number comes after the unit; the second Mach number is "Mach 2" instead of "2 Mach" (or Machs). This is somewhat reminiscent of the early modern ocean sounding unit "mark" (a synonym for fathom), which was also unit-first, and may have influenced the use of the term Mach. In the decade preceding man's flying faster than sound, aeronautical engineers referred to the speed of sound as Mach's number, never "Mach 1