Subsonic
Also: sub-sonic
A load whose bullet stays below the speed of sound, roughly 1,125 fps at sea level, for its entire flight. Common in suppressed and .22 LR shooting.
The speed of sound is near 1,125 feet per second at sea level, and it shifts with temperature and altitude. A subsonic load is built to stay under that number from muzzle to target, which avoids the sharp crack a bullet makes when it breaks the sound barrier. Paired with a suppressor, a true subsonic round is genuinely quiet.
The catch for distance work is the transonic zone. A bullet that launches just above the sound barrier and decelerates back through it often gets buffeted and loses stability, scattering groups. Dedicated subsonic shooters sidestep that by staying under the speed of sound the entire way, then accept steep bullet drop and short effective range.