An aircraft whose maximum speed is less than the speed of sound

(of a sound, scientific) having a frequency too low to be audible

(of a sound, literary) very quiet, almost inaudible

“The noise of the ship was all around her. The faint subsonic rumble of the reactor and drive. (Leviathan Wakes, James S. A. Corey)”

(of a speed) less than the speed of sound (in the same medium, and at the same temperature and pressure)