Mach

Mach is a unit of measurement used to describe the speed of an object relative to the speed of sound in the surrounding medium, typically air. For example, Mach 1 is the speed of sound, about 761 miles per hour (1,225 kilometers per hour) at sea level. If an airplane travels at Mach 2, it's moving twice as fast as sound. The Mach number varies depending on temperature and altitude because the speed of sound changes with atmospheric conditions. Mach is commonly used in aeronautics and aerospace to specify high-speed aircraft and spacecraft velocities.