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.

Citrus is a genus of flowering trees and shrubs in the rue family, Rutaceae. Plants in the genus produce citrus fruits, including crops such as oranges, mandarins, lemons, grapefruits, pomelos, and limes. The genus Citrus is native to South Asia, East Asia, Southeast Asia, Melanesia, and Australia. Various citrus species have been used and domesticated by indigenous cultures in these areas since ancient times. From there its cultivation spread into Micronesia and Polynesia by the Austronesian expansion ; and to the Middle East and the Mediterranean via the incense trade route, and onwards to Europe and the Americas.