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 tangerine (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. Oranges originated in Southeast Asia. The fruit of Citrus sinensis is called sweet orange to distinguish it from Citrus aurantium, the bitter orange.

In a number of languages, it is known as a "Chinese apple" (e.g. Dutch Sinaasappel, "China's apple", or "Apfelsine" in German). The name is thought to ultimately derive from the Dravidian and Telugu word for the orange tree, with its final form developing after passing through numerous intermediate languages. 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 and 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.

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