A Clementine (Citrus × clementina) is a hybrid between a mandarin orange and a sweet orange so named in 1902. Clementine and mandarin oranges are members of the citrus family just like traditional Oranges, but they each taste slightly different. The Clementine is not always easy to distinguish from varieties of Mandarins but through sampling you can clearly taste a difference. Clementine oranges look like tiny versions of regular oranges, and they have a tart, tangy and rich sweet flavour. The exterior is a deep orange colour with a smooth, glossy appearance. Clementines can be separated into 7 to 14 segments. They tend to be easy to peel. Clementines are a type of citrus called zipper-peel, which means the skin comes off very easily. They are almost always seedless when grown commercially (without cross-pollination). Their oils, like other citrus fruits, contain mostly limonene as well as myrcene, linalool, α-pinene and many complex aromatics.
Clementines are a highly important North African variety originated as an accidental hybrid in a planting of mandarin seedlings, presumably of the common or Mediterranean mandarin, made by Father Clement Rodier (after whom the fruit was named) in the garden of an orphanage at Misserghin, a small village near Oran, Algeria. It is assumed that the seed parent was the Mediterranean mandarin and the pollen parent a willow-leafed ornamental variety of C. aurantium known as Granito. However, there are claims it originated in China much earlier; one source describes it as nearly identical to the Canton mandarin widely grown in the Guangxi and Guangdong provinces in China.
This variety was introduced into California commercial agriculture in 1914, though it was grown at the Citrus Research Center (now part of the University of California, Riverside) as early as 1909. Clementines lose their desirable seedless characteristic when they are cross-pollinated with other fruit. To prevent this, in 2006 growers such as Paramount Citrus in California threatened to sue local beekeepers to keep bees away from their crops.
For further reading here is an interesting article about Clementines for any hardcore Citrus Nerds by the University of California at Riverside (Citrus Variety Collection).