The Key lime is a citrus fruit . The Key lime ( Citrus aurantifolia ) is a citrus species with a globose fruit , 2.5 â `` 5 cm in diameter ( 1 â `` 2 in ) , that is yellow when ripe but usually picked green commercially . It is smaller , seedier , has a higher acidity , a stronger aroma , and a thinner rind than that of the Persian lime ( Citrus x latifolia ) .