A key lime is a hybrid citrus hybrid fruit that is spherical and measures about 1 to 2 inches across. It is green when picked but becomes yellow when ripe. A ripe key lime feels heavy for its size. Compared to a Persian lime, it is a smaller and has more seeds to remove, higher acidity, a stronger citrus aroma, and a thinner rind. The key lime is valued for its unique flavor and costs more too.