Definition of "subsonic"

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

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

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

(fluid mechanics, of a flow) with a Mach number such that 0.3 < Ma < 0.8 (approximately - the exact limits vary between sources)  examples

an aircraft whose maximum speed is less than the speed of sound  examples