subsonic
noun
Definitions
An aircraft whose maximum speed is less than the speed of sound
adjective
Definitions
(of a sound, scientific) having a frequency too low to be audible
(of a sound, literary) very quiet, almost inaudible
Example: 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)
(of a flow) with a Mach number such that 0.3 < Ma < 0.8 (approximately - the exact limits vary between sources)