sitz bath vs bathtub

sitz bath and bathtub both are nouns.

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

As nouns, bathtub is a hypernym of sitz bath; that is, bathtub is a word with a broader meaning than sitz bath:

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
bathtub: a relatively large open container that you fill with water and use to wash the body

Other hypernyms of sitz bath include bathing tub, tub.

sitz bath (noun)	bathtub (noun)
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	a relatively large open container that you fill with water and use to wash the body