bathtub vs tub

bathtub and tub both are nouns.

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

As nouns, bathtub and tub are synonyms defined as:

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

Other synonyms of bathtub include bathing tub.

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