bitter orange tree vs bitter orange

bitter orange tree and bitter orange both are nouns.

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

As nouns, bitter orange tree and bitter orange are synonyms defined as:

bitter orange tree and bitter orange: any of various common orange trees yielding sour or bitter fruit; used as grafting stock

Other synonyms of bitter orange tree include bigarade, Citrus aurantium, marmalade orange, Seville orange, sour orange.

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