noumenon vs thing-in-itself

noumenon and thing-in-itself both are nouns.

Word	Noun	Adjective	Verb	Adverb
noumenon	Yes	No	No	No
thing-in-itself	Yes	No	No	No

As nouns, noumenon and thing-in-itself are synonyms defined as:

noumenon and thing-in-itself: the intellectual conception of a thing as it is in itself, not as it is known through perception

noumenon (noun)	thing-in-itself (noun)
the intellectual conception of a thing as it is in itself, not as it is known through perception	the intellectual conception of a thing as it is in itself, not as it is known through perception