A Mach number is a ratio of an object's velocity divided by the speed of sound in the medium the object is traveling through. Mach number is used to measure an object's speed with respect to the medium and conditions surrounding it. For example, a Mach number of 1 means the object is traveling at the speed of sound. Anything greater than 1 and the object will have passed through the sound barrier and is now traveling faster than the speed of sound.