Sweet orange is refers to the citrus tree 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 angiosperm tree , growing to about 10 m tall, with thorny shoots and evergreen leaves , which are arranged alternately, of ovate shape with crenulate margins and 4-10 cm long. The orange fruit is a hesperidium .

The word "orange" ultimately comes from Sanskrit narang . Oranges originated in southeast Asia , in either India , Vietnam or southern China . 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).

Source: http://en.wikipedia.org/wiki/Orange_fruit