bathtub vs sitz bath

bathtub and sitz bath both are nouns.

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

As nouns, sitz bath is a hyponym of bathtub; that is, sitz 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
sitz 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, hip bath.

bathtub (noun)	sitz 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