In aeronautics, the Mach number M is the ratio between the speed of an object moving through a medium and the speed of sound of that medium. Mach 1 represents the speed of sound in air, which is approximately 330 meters per second, or 760 miles per hour.
Flights with M less than 1 occur at speeds lower than the speed of sound and are called subsonic. Similarly, an M value of 1 describes transonic flights, while M values between 1 and 5 define supersonic flights. Aircrafts flying at speeds above Mach 3 typically need to deal with intense heat due to air friction. This becomes a greater issue for flights at more than 5 times the speed of sound, which are also called hypersonic.