bathing tub vs bathtub

bathing tub and bathtub both are nouns.

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

As nouns, bathing tub and bathtub are synonyms defined as:

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

Other synonyms of bathing tub include tub.

bathing tub (noun)	bathtub (noun)
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