Definiciones
1.Adjectivehaving a frequency too low to be audible
2.Adjectivevery 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)"
3.Adjectiveless than the speed of sound (in the same medium, and at the same temperature and pressure)
4.Adjectivewith a Mach number such that 0.3 < Ma < 0.8 (approximately - the exact limits vary between sources)
5.Nounan aircraft whose maximum speed is less than the speed of sound