The orange (specifically, the sweet orange) is the fruit of the citrus species Citrus × sinensis in the family Rutaceae.

The fruit of the Citrus × sinensis is considered a sweet orange, whereas the fruit of the Citrus × aurantium is considered a bitter orange. The sweet orange reproduces asexually (apomixis through nucellar embryony); varieties of sweet orange arise through mutations.

Sweet oranges were mentioned in Chinese literature in 314 BC. As of 1987, orange trees were found to be the most cultivated fruit tree in the world. Orange trees are widely grown in tropical and subtropical climates for their sweet fruit. The fruit of the orange tree can be eaten fresh, or processed for its juice or fragrant peel. As of 2012, sweet oranges accounted for approximately 70% of citrus production.

In 2013, 71.4 million metric tons of oranges were grown worldwide, production being highest in Brazil and the U.S. states of Florida and California.

All citrus trees belong to the single genus Citrus and remain almost entirely interfertile. This means that there is only one superspecies that includes grapefruits, lemons, limes, oranges, and various other types and hybrids.[9] As the interfertility of oranges and other citrus has produced numerous hybrids and cultivars, and bud mutations have also been selected, their taxonomy is fairly controversial, confusing or inconsistent.[7][10] The fruit of any citrus tree is considered a hesperidium, a kind of modified berry; it is covered by a rind originated by a rugged thickening of the ovary wall.

Different names have been given to the many varieties of the genus. Orange applies primarily to the sweet orange – Citrus sinensis (L.) Osbeck. The orange tree is an evergreen, flowering tree, with an average height of 9 to 10 m (30 to 33 ft), although some very old specimens can reach 15 m (49 ft).[13] Its oval leaves, alternately arranged, are 4 to 10 cm (1.6 to 3.9 in) long and have crenulate margins.[14] Although the sweet orange presents different sizes and shapes varying from spherical to oblong, it generally has ten segments (carpels) inside, and contains up to six seeds (or pips)[15] and a porous white tissue – called pith or, more properly, mesocarp or albedo—lines its rind.[16] When unripe, the fruit is green. The grainy irregular rind of the ripe fruit can range from bright orange to yellow-orange, but frequently retains green patches or, under warm climate conditions, remains entirely green. Like all other citrus fruits, the sweet orange is non-climacteric. The Citrus sinensis is subdivided into four classes with distinct characteristics: common oranges, blood or pigmented oranges, navel oranges, and acidless oranges.

The orange is a hybrid, between pomelo (Citrus maxima) and mandarin (Citrus reticulata). It has genes that are ~25% pomelo and ~75% mandarin; however, it is not a simple backcrossed BC1 hybrid, but hybridized over multiple generations.[4] The chloroplast genes, and therefore the maternal line, seem to be pomelo. The sweet orange has had its full genome sequenced.[2] Earlier estimates of the percentage of pomelo genes varying from ~50% to 6% have been reported.

Other citrus groups also known as oranges are:

Bitter orange (Citrus aurantium), also known as Seville orange, sour orange (especially when used as rootstock for a sweet orange tree), bigarade orange and marmalade orange. Like the sweet orange, it is a pomelo x mandarin hybrid.

Bergamot orange (Citrus bergamia Risso), grown mainly in Italy for its peel, producing a primary essence for perfumes, also used to flavor Earl Grey tea. It is a hybrid, probably bitter orange x limetta.

Trifoliate orange (Poncirus trifoliata), sometimes included in the genus (classified as Citrus trifoliata). It often serves as a rootstock for sweet orange trees and other Citrus cultivars.

Mandarin orange (Citrus reticulata) is an original species of citrus, and is a progenitor of the common orange. An enormous number of cultivars have, like the sweet orange, a mix of pomelo and mandarin ancestry. Some cultivars are mandarin-pomelo hybrids, bred from the same parents as the sweet orange (e.g. the tangor and ponkan tangerine). Other cultivars are sweet orange x mandarin hybrids (e.g. clementines). Mandarin traits generally include being smaller and oblate, easier to peel, and less acidic.

Pomelo traits include a thick white albedo (rind pith, mesocarp) that is more closely attached to the segments.

Orange trees generally are grafted. The bottom of the tree, including the roots and trunk, is called rootstock, while the fruit-bearing top has two different names: budwood (when referring to the process of grafting) and scion (when mentioning the variety of orange).