Mach was an Austrian physicist who gave his name to the Mach number. This number is a method of relating an object's speed to the speed of sound in a particular medium, usually air.
He also worked on shock waves and inertia, coming to conclusions that conflicted with Einsteins' theory of relativity.
Below Mach1 called subsonic.
Above Mach1 called Supersonic
Above Mach5 called hypersonic