Calamansi (Citrus × microcarpa), also known as calamondin or Philippine lime. It is native to the Philippines and surrounding areas in southern China, Taiwan, Borneo, and Sulawesi. It is used in various condiments, beverages, dishes, marinades, and preserves.
Calamansi is a hybrid between kumquat and another species of Citrus (in this case probably the mandarin orange).