Citrus is a common term and genus (Citrus) of flowering plants in the rue family, Rutaceae. Citrus is believed to have originated in the part of Southeast Asia bordered by Northeastern India, Myanmar (Burma) and the Yunnan province of China. Citrus fruit has been cultivated in an ever-widening area since ancient times; the best-known examples are the oranges, lemons, grapefruit, and limes.

The generic name originated in Latin, where it specifically referred to the plant now known as Citron (C. medica). It was derived from the ancient Greek word for cedar, κέδρος (kédros). Some believe this was because Hellenistic Jews used the fruits of C. medica during Sukkot (Feast of the Tabernacles) in place of a cedar cone, while others state it was due to similarities in the smell of citrus leaves and fruit with that of cedar. Collectively, Citrus fruits and plants are also known by the Romance loanword agrumes (literally "sour fruits").

The taxonomy and systematics of the genus are complex and the precise number of natural species is unclear, as many of the named species are hybrids clonally propagated through seeds (by apomixis), and there is genetic evidence that even some wild, true-breeding species are of hybrid origin. Cultivated Citrus may be derived from as few as four ancestral species. Natural and cultivated origin hybrids include commercially important fruit such as the oranges, grapefruit, lemons, some limes, and some tangerines.