Which citrus fruit is the key ingredient in Key lime pie?

The citrus fruit that is the key ingredient in Key lime pie is the Key lime.

Key limes are small, thin-skinned, aromatic limes also known as West Indian or Mexican limes. Their juice is more acidic and intensely fragrant than the juice of the larger Persian limes commonly sold in American supermarkets. Ripe Key limes produce pale yellow juice, which helps explain why an authentic filling can look yellow rather than bright green.

The Florida dessert combines Key lime juice with egg yolks and sweetened condensed milk, usually in a graham-cracker crust. The acidic juice reacts with proteins in the egg yolks and condensed milk, thickening the filling; baking is commonly used to pasteurize the eggs and set it further.

The pie is strongly associated with Key West, although its precise origin is debated. Documented versions appeared by the 1930s and 1940s, while the popular story that a cook named Aunt Sally invented it in the 19th century lacks early supporting evidence. Persian limes can make a tasty substitute, but they are not the named traditional ingredient.