Transonic
Also: Transonic Range
The speed band around the sound barrier, roughly Mach 1.2 down to 0.8, where shifting airflow can disturb a bullet's flight.
The transonic range is the narrow speed band on either side of the speed of sound, where a bullet is neither cleanly supersonic nor settled into subsonic flight. As the bullet decelerates through this zone the shock waves around it move and collapse, and the resulting pressure shifts can buffet a projectile with a modest stability margin.
This is why long range shooters care about where their bullet goes transonic. A projectile with a comfortable stability factor usually rides through the zone with little drama, but one with only a slim stability margin can yaw or wander as it slows. Knowing the distance at which a load enters the transonic region tells you the realistic ceiling for predictable accuracy.