A bathtub is a long, large container that you fill with water and sit or lie in to wash yourself. You take a bath in a bathtub; you take a shower standing up. In American English this is the usual word for the container. In British English people often just call it a bath — same object, shorter name. Old houses may have a deep tub on little feet. The word is only for this washing container, not for a swimming pool, a sink, or a bucket. Though actually, tiny homes sometimes have no bathtub at all, only a shower.