tub vs footbath

tub and footbath both are nouns.

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

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

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 tub include hot tub, hip bath, sitz bath.

tub (noun)	footbath (noun)
a large open vessel for holding or storing liquids	a small bathtub for warming or washing or disinfecting the feet
a relatively large open container that you fill with water and use to wash the body
the amount that a tub will hold