An orange—specifically, the sweet orange—is the citrus Citrus × sinensis (syn. Citrus aurantium L. var. dulcis L., or Citrus aurantium Risso) and its fruit. The orange is a hybrid of ancient cultivated origin, possibly between pomelo (Citrus maxima) and mandarin (Citrus reticulata). It is a small flowering tree growing to about 10 m tall with evergreen leaves, which are arranged alternately, of ovate shape with crenulate margins and 4–10 cm long. The orange fruit is a hesperidium, a type of berry.
All citrus trees are of the single genus, Citrus, and remain largely interbreedable; that is, there is only one "superspecies" which includes grapefruits, lemons, limes, and oranges. Nevertheless, names have been given to the various members of the genus, oranges often being referred to as Citrus sinensis and Citrus aurantium. Fruits of all members of the genus Citrus are considered berries because they have many seeds, are fleshy, soft, and derive from a single ovary. An orange seed is called a pip. The white thread-like material attached to the inside of the peel is called pith.
The fruit has found a niche as an interesting ingredient variation on traditional Seville marmalade, with its striking red streaks and distinct flavour. The scarlet navel is a variety with the same dual-fruit mutation as the navel orange.
According to Dorsett, Shamel, and Popenoe (1917) of the U.S. Department of Agriculture who conducted a study at first hand, a single mutation in 1810 to 1820 in a Selecta orange tree planted at a monastery near Bahia in Brazil, probably yielded the navel orange, also known as the Washington, Riverside, or Bahia navel. However, a researcher at the University of California, Riverside, believes that the parent variety was more likely the Portuguese navel (Umbigo) orange described by Risso and Poiteau (1818-22). The mutation causes the orange to develop a second orange at the base of the original fruit, opposite the stem, as a conjoined twin in a set of smaller segments embedded within the peel of the larger orange. From the outside, it looks similar to the human navel, hence its name.