The bitter orange is perhaps best known as the Seville orange, mostly for marketing purposes. However, this refers to a specific cultivar, and not all bitter oranges are Seville oranges. There are a number of other common names, including the sour orange, bigarade orange, and the marmalade orange. It is, as the name suggests, less sweet than the more common sweet orange, and is only used as a flavoring or ingredient rather than as a stand-alone fruit to snack upon.
The orange peel is used to produce essential oil for perfumes, in marmalade (it is higher in pectin than sweet oranges), in compotes and for orange-flavored liqueurs, making bigarade sauce (bigarade being the French word for bitter orange) for duck l'orange, Chinese ponzu sauce, and is used dried as a seasoning for recipes around the world.
The tree itself is often used as a grafting tree, especially as rootstock, as they have good cold hardiness, (mostly) good disease resistance, and are said to give a good flavor to the resulting fruit. This may be changing, however, as bitter orange is highly susceptible to the Citrus tristeza virus, which is currently causing fruit growers trouble.
Bitter orange has long been touted as having medical properties, but has been found to be harmful in many cases, and should not be taken in large doses, and ideally, should not be used medicinally at all. Products produced from various varieties of bitter orange include petitgrain, neroli oil, bergamot oil, kijitsu (枳実), tohi (橙皮), and orange flower water. There are no serious harmful effects reported from the small amounts found in bergamot flavored teas (e.g. Earl Grey) or foods (e.g., marmalade and Turkish delight), although in large amounts these might cause muscle cramps.
Bitter orange is believed to be a hybrid of the pomelo and the mandarin orange. As such is does not have a species name, but rather a hybrid name: Citrus ×aurantium, sometimes written in full as C. maxima × C. reticulata.