Mach number (Ma) 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

Mach number is the number of times the speed of sound an object or a duct, or the fluid medium itself, move relative to each other. It is named after Austrian physicist and philosopher Ernst Mach. Unlike most units of measure, with Mach the number comes after the unit, so one says "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 flying faster than sound, aeronautical engineers referred to the speed of sound as Mach's number, never "Mach 1".

High speed flight can be roughly classified in five categories:

• sonic: Ma=1
• Subsonic: Ma < 1
• Transonic: 0.8 < Ma < 1.2
• Supersonic: 1.2 < Ma < 5
• Hypersonic: Ma > 5