bitter orange vs orange tree

bitter orange and orange tree both are nouns.

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

As nouns, orange tree is a hypernym of bitter orange; that is, orange tree is a word with a broader meaning than bitter orange:

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

Other hypernyms of bitter orange include orange.

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