Original: The Key lime (Citrus aurantifolia) is a citrus species with a globose fruit, 2.5-5 cm in diameter (1-2 in), that is yellow when ripe but usually picked green commercially.
Simplified: The Key lime is a citrus fruit. Its fruit is 2.5-5 cm in diameter (1-2 in). The fruit is  yellow when ripe but usually it is picked green.