bitter orange

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An orange tree (Citrus aurantium) that is widely cultivated in warm regions and has reddish-orange, highly acidic fruit.
• noun The fruit of this tree.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A citrus tree of the species Citrus aurantium.
• noun Fruit of the species Citrus aurantium.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

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