What is the meaning of Subsonic?

having a frequency too low to be audible

very quiet, almost inaudible

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

with a Mach number such that 0.3 < Ma < 0.8 (approximately - the exact limits vary between sources)

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

subsonic

Source: wiktionary.org