In fluid mechanics, Mach number (Failed to parse (Missing texvc executable; please see math/README to configure.): \mathrm{Ma}

) (generally  /ˈmɑːk/, sometimes /ˈmɑːx/ or /ˈmæk/) is a dimensionless number representing the speed of an object moving through air or other fluid divided by the local speed of sound. It is commonly used to represent the speed of an object when it is traveling close to or above the speed of sound.

where

Mach number varies by the composition of the surrounding medium and also by local conditions, especially temperature and pressure. The Mach number can be used to determine if a flow can be treated as an incompressible flow. If M < 0.2–0.3 and the flow is (quasi) steady and isothermal, compressibility effects will be small and a simplified incompressible flow model can be used.

The Mach number is named after Austrian physicist and philosopher Ernst Mach, a designation proposed by aeronautical engineer Jakob Ackeret. 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 faster-than-sound human flight, aeronautical engineers referred to the speed of sound as Mach's number, never "Mach 1."