Subsonic means any body (monkey poo, berets, Chav after a giro or whatever) having a velocity less than the speed of sound.
The speed of sound is effected by the medium it is being transmitted in (air, water etc.) and its density. The density may be affected by temperature(see headache, can of worms and failed exams).
Subsonic is ususally used to describe the speed of ammunition rounds, aircraft and the like. To break the speed of sound barrier you need to be travelling at around 660 mph (at about 36,000 feet) to around 760 mph (at Sea level).