bathing tub vs footbath

bathing tub and footbath both are nouns.

Word	Noun	Adjective	Verb	Adverb
bathing tub	Yes	No	No	No
footbath	Yes	No	No	No

As nouns, footbath is a hyponym of bathing tub; that is, footbath is a word with a more specific, narrower meaning than bathing tub:

bathing tub: 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 bathing tub include hot tub, hip bath, sitz bath.

bathing tub (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