An orange, also called sweet orange to distinguish it from the bitter orange, is the fruit of a tree in the family Rutaceae. Botanically, this is the hybrid Citrus × sinensis, between the pomelo and the mandarin orange. The chloroplast genome, and therefore the maternal line, is that of pomelo. The sweet orange has had its full genome sequenced.

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.