bitter orange vs bitter orange

bitter orange and bitter orange both are nouns.

Word	Noun	Adjective	Verb	Adverb
bitter orange	Yes	No	No	No
bitter orange	Yes	No	No	No

bitter orange (noun)	bitter orange (noun)
highly acidic orange used especially in marmalade	highly acidic orange used especially in marmalade
any of various common orange trees yielding sour or bitter fruit; used as grafting stock	any of various common orange trees yielding sour or bitter fruit; used as grafting stock