The label citrus refers to the common name and the genus Citrus in the Rutaceae (rue family of flowering plants). Citrus plants are believed to have originated in an area bordered by NE India, Myanmar/Burma, and Yunnan (China); but are currently cultivated around the world in generally warm weather climates without frosts.
Citrus trees include oranges, lemons, grapefruits, and limes. Research suggests that Fortunella (kumquats), Poncirus, Microcitrus and Eremocitrus should also be included in Citrus. Plants from the Clymenia will also hybridize with kumquats and limes, suggesting these are also closely related.
Citrus plants are typically grown as large shrubs or small trees with spiny shoots and evergreen leaves. Citrus fruits are "hesperidium" = a specialized kind of berry with a leathery rind and segments of pulp (liquid-filled) vesicles. Fruit is usually eaten fresh or pressed for juice. Citrus fruit and juice are good sources of vitamin C, flavonoids, and citric acid (ascorbic acid). The citric acid gives citrus fruit a characteristic sharp flavor.
For cultivation, citrus trees hybridize easily, and are often propagated with grafts to preserve a variety's characteristics. Most require non-freezing temperatures, although some oranges (especially mandarin oranges and the related Trifoliate Orange) have some frost tolerance. Frosts tend to result in particularly sour fruit.