What is the Mach number ?

The Mach number is a value which represents the ratio of the speed of an object to the speed of sound.

The speed of sound depends on parameters such as density and temperature.  The Mach number is named after Ernst Mach, an Austrian physicist.  If the Mach number is less than 1, it implies that the speed of the object is lesser than the speed of sound.  When the speed of the object exceeds the speed of sound, the Mach number will be greater than 1.

For instances, supersonic aircraft which fly at speeds above the speed of sound have a Mach number that is greater than 1.

When the speed of the flow or of an object is equal to the speed of sound, the flow is said to be sonic flow.  If it is less than one it is subsonic and if it is greater than 1 it is supersonic.