basement

A basement is the room or floor under a house, below ground level. In the basement, a basement flat, the basement flooded.

Cellar is the older British word and tends to mean a smaller storage space, while a basement is a proper room and often a whole floor. Basement flats are cheaper and darker for the same reason. In shops, the basement is a floor, and bargain basement means cheap and low-quality. Note that basements are rare in newer British housing and standard in America, which is a difference of building tradition and water table.