Supersonic means faster than the speed of sound. When something travels at supersonic speed, it moves so fast that it outraces the sound waves it creates.

Sound travels through air at about 767 miles per hour (that's the speed at sea level on a typical day). Anything moving faster than that is supersonic. Fighter jets can fly at supersonic speeds, and when they break through the sound barrier, people on the ground hear a loud boom called a sonic boom. That boom happens because the aircraft compresses air waves as it catches up to its own sound.

The Concorde, a passenger airplane that flew from 1976 to 2003, could carry people across the Atlantic Ocean at supersonic speeds, cutting flight time in half. A bullet fired from most guns is supersonic, which is why you can't hear it coming.

Scientists and engineers often use “Mach numbers” to describe supersonic speeds: Mach 1 equals the speed of sound, so Mach 2 means twice the speed of sound, and so on. Anything slower than sound is called subsonic.