tub vs bathtub

tub and bathtub both are nouns.

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

As nouns, tub and bathtub are synonyms defined as:

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

Other synonyms of tub include bathing tub.

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