bathtub vs hip bath

bathtub and hip bath both are nouns.

Word	Noun	Adjective	Verb	Adverb
bathtub	Yes	No	No	No
hip bath	Yes	No	No	No

As nouns, hip bath is a hyponym of bathtub; that is, hip bath is a word with a more specific, narrower meaning than bathtub:

bathtub: a relatively large open container that you fill with water and use to wash the body
hip bath: a bathtub in which your buttocks and hips are immersed as if you were sitting in a chair and you bathe in a sitting position

Other hyponyms of bathtub include footbath, hot tub, sitz bath.

bathtub (noun)	hip bath (noun)
a relatively large open container that you fill with water and use to wash the body	a bathtub in which your buttocks and hips are immersed as if you were sitting in a chair and you bathe in a sitting position