Slower than the speed of sound.

See also: supersonic

Cartridges that are designed to be subsonic are quieter, especially when used with a sound suppressor, because they do not produce a sonic crack.

As a bullet travels through the air, it loses velocity due to air drag. If a bullet crosses the threshold between supersonic and subsonic, an element of instability in the bullet’s spin is introduced, which results in reduced accuracy. Some ammunition (most commonly .22 Long Rifle target ammunition) is designed to be subsonic to avoid contributing to inaccuracy in this manner.