Definition (Source: Wikipedia)
***A basement is one or more floors of a building that are either completely or partially below the ground floor. Slab-on-grade buildings do not have basements. Basements are typically used as a utility space for a building where such items as the furnace, water heater, car park, and air-conditioning system are located; so also are amenities such as the electrical distribution system, and cable television distribution point.
In British English the word 'basement' is rarely used and the word 'cellar' is used to apply to any such large underground room.