The Mach number (M) refers to the method of measuring airspeed that was developed by the Austrian physicist Ernst Mach. It is used to indicate flight velocities in high-speed flight and is related to the speed of sound. The actual speed of sound varies depending on the altitude above sea level because sound travels at slightly different speeds at different temperatures, and the temperature varies according to altitude. At sea level, the speed of sound is about 761 miles per hour (1,225 kilometers per hour). At 20,000 feet (6,096 meters), the speed of sound is 660 miles per hour (1,062 kilometers per hour).

If an aircraft is traveling at one half the speed of sound, it is said to be traveling at Mach 0.5. A speed of Mach 2 is twice the speed of sound. Because the speed of sound varies, a particular speed at sea level expressed as a Mach number would be faster than the same speed at 30,000 feet (9,144 meters), which would be faster than the same speed at 40,000 feet (12,192 meters). In other words, Mach 2 at sea level is a greater number of miles per hour (or kilometers per hour) than Mach 2 at 30,000 feet, which is a greater number of miles per hour than Mach 2 at 40,000 feet. When an aircraft reaches Mach 1, it is said to "break the sound barrier."

The following breakdowns have been generally accepted to classify speeds:
M less than 0.8 subsonic
M = 0.8 to 1.2 transonic
M - 1.2 to 5.0 supersonic
M greater than 5.0 hypersonic

A "critical Mach number" is the speed of an aircraft (below Mach 1) when the air flowing over some area of the airfoil has reached the speed of sound. For instance, if the air flowing over a wing reaches Mach 1 when the wing is only moving at Mach 0.8, then the wing's critical Mach number is 0.8.