Bitter orange, sour orange, Seville orange, bigarade orange, or marmalade orange is in a narrow sense the citrus tree Citrus × aurantium and its fruit. It is native to Southeast Asia and has been spread by humans to many parts of the world. It is probably a cross between the pomelo, Citrus maxima, and the mandarin orange, Citrus reticulata.

Marmalade is a fruit preserve made from the juice and peel of citrus fruits boiled with sugar and water. The well-known version is made from bitter orange. It is also made from lemons, limes, grapefruits, mandarins, sweet oranges, bergamots, and other citrus fruits, or a combination. Citrus is the most typical choice of fruit for marmalade, though historically the term has often been used for non-citrus preserves.