Supersonic
Faster than the speed of sound, the flight regime where most rifle bullets travel until drag slows them back toward the sound barrier.
A bullet is supersonic whenever it is moving faster than the local speed of sound, which is roughly 1,125 feet per second at sea level and varies with temperature. Nearly every centerfire rifle round leaves the barrel well above that mark, since typical muzzle velocity figures run two to three times the speed of sound.
Staying supersonic matters because flight is most predictable while the bullet remains comfortably above Mach 1. As drag erodes speed, the bullet eventually crosses into the transonic band and then into subsonic flight, where trajectories grow harder to model. The supersonic range of a load effectively defines the distance over which it stays easy to shoot accurately.