Citrus is a genus of flowering trees and shrubs in the rue family, Rutaceae. This includes any small tree or spiny shrub, including the lemon, lime, orange, tangerine, grapefruit, citron, kumquat, and shaddock, widely cultivated for fruit or grown as an ornamental.

It is believed that citrus plants are native to subtropical and tropical regions of Asia and the Malay Archipelago. Some citrus species have been present in the Mediterranean basin for centuries. Since citrus originates from these warmer climates they are not frost hardy. Temperatures not falling below −2 °C (28 °F) are required for successful cultivation. Theses trees thrive in a consistent sunny, humid environment with fertile soil and adequate rainfall or irrigation.

Today, in the United States, the most important commercial varieties include oranges, grapefruit, lemons, and tangerines and to a lesser extent, tangelos. Oranges account for the greatest value in terms of production; followed by grapefruit, lemons and tangerines