subsonic

This term is primarily used in aeronautics and physics to describe speeds below Mach 1. It carries a technical, precise connotation and is rarely used in casual conversation unless referring to specific aircraft capabilities or atmospheric phenomena.

In a comparative context, it is the direct opposite of supersonic (faster than sound) and transonic (near the speed of sound). While "slow" is a general descriptor, "subsonic" specifically anchors the speed to the physical constant of the speed of sound, making it a term of scientific measurement rather than subjective perception.