Refers to the physical tub itself, not the bathroom or the act of bathing.
新しい浴槽を設置した。
We installed a new bathtub.
浴槽にお湯をためる。
Fill the bathtub with hot water.
I filled the bathtub with hot water before getting in.
I was tired, so I soaked slowly in the bathtub.
I gave the bathtub a good scrub on the weekend.