The Key lime (Citrus × aurantiifolia) is a citrus hybrid (C. micrantha x C. medica) with a spherical fruit, 25–50 mm (1–2 in) in diameter. The Key lime is usually picked while it is still green, but it becomes yellow when ripe.
The Key lime is smaller, seedier, has higher acidity, stronger aroma, and thinner rind than the Persian lime (Citrus × latifolia). The Key lime is valued for its unique flavor compared with other limes. The name is derived from its association with the Florida Keys, where it is best known as the flavoring ingredient in Key lime pie. It is also known as West Indian lime, bartender's lime, Omani lime, or Mexican lime, the last classified as a distinct race with a thicker skin and darker green colour. Philippine varieties have various names, including dayap and bilolo.