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.
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. 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.
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.