bathtub vs footbath

bathtub and footbath both are nouns.

Word	Noun	Adjective	Verb	Adverb
bathtub	Yes	No	No	No
footbath	Yes	No	No	No

As nouns, footbath is a hyponym of bathtub; that is, footbath 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
footbath: a small bathtub for warming or washing or disinfecting the feet

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

bathtub (noun)	footbath (noun)
a relatively large open container that you fill with water and use to wash the body	a small bathtub for warming or washing or disinfecting the feet