The term supersonic speed unit is often abbreviated as MACH, which is a four letter word. MACH stands for 'Mach number', which is a measure of the speed of an object relative to the speed of sound in the surrounding medium. It is named after Austrian physicist Ernst Mach, who first proposed the concept in 1877. The Mach number is used to describe the speed of aircraft, missiles, and other objects traveling through air or other gases.