bathtub vs bathing tub

bathtub and bathing tub both are nouns.

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

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

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

Other synonyms of bathtub include tub.

bathtub (noun)	bathing tub (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