The Key lime is a citrus fruit. Its Latin name is Citrus aurantiifolia (often, less correctly: C. aurantifolia), or Citrus x aurantiifolia (Christm.) Swingle. It is also known as the Mexican lime, West Indian lime or Bartender's lime. It is a shrub that grows to about 5m in height.
Its fruit is 2.5-5 cm in diameter (1-2 in). The fruit is yellow when ripe but usually it is picked green. It is smaller, seedier, has a higher acidity, a stronger aroma, and a thinner rind than that of the more common Persian lime. It is valued for its unique flavor compared to other lime. The key lime usually has a more tart and bitter flavor. It is perhaps most distinguished as an ingredient in the Key lime pie.
C. aurantiifolia is a shrubby tree, that grows to 5 m (16 ft), with many thorns. Dwarf varieties are popular with home growers and can be grown indoors in winter in colder climates. The trunk rarely grows straight, with many branches that often originate quite far down on the trunk. The leaves are ovate 2.5–9 cm (1–3.5 in) long. They resemble orange leaves (the scientific name aurantiifolia refers to the leaves' resemblance). The flowers are 2.5 cm (1 in) in diameter. They are yellowish white with a light purple tinge on the margins. Flowers and fruit appear throughout the year but are most abundant from May to September  .
C. aurantiifolia originally came from Southeast Asia. It was introduced through the Middle East to North Africa and Europe during the Crusades. Spanish explorers then took it to the West Indies (at some point including the Florida Keys) contemporaneously with Columbus, then tropical and sub-tropical North America including Mexico, Florida, and later California . The English name "lime" was derived from the Persian name لیمو Limu in this course. "Key" would seem to have been added some time after the Persian lime cultivar became more important commercially in the United States. This was after the hurricane of 1926. The hurricane destroyed most U.S. C. aurantiifolia plants. Some plants growing in the wild in the Keys survived the hurricane. Since the North American Free Trade Agreement came into effect, many Key limes are grown in Mexico and Central America. They are also grown in Texas and California.