The Mach number is a dimensionless quantity representing the ratio of the speed of an object to the speed of sound in the surrounding medium. It is named after the Austrian physicist Ernst Mach.

Mach 1 represents the speed of sound. Speeds below Mach 1 are "subsonic," while speeds above Mach 1 are "supersonic." Speeds above Mach 5 are typically referred to as "hypersonic."