In the Philippines, we call this fruit calamansi. From a citrus family, calamansi is a fruit which is widely cultivated in the Philippines. In our dialect, we call it limonsito. Here in the US, they call it Calamondin and sometimes, they call it golden lime, panama orange, chinese orange, acid orange, calamonding, or calamandarin. It was believed to originate in China and spread throughout Southeast Asia, India as well as Hawaii, the West Indies, Central and North America. the photo below is my calamondin plant which I bought at Wal-Mart for $9 and been producing fruits more so often.
Calamansi has white or purplish flowers. Its fruit has a spongy/leathery rind with a juicy pulp that is divided into sections. When I am making a Filipino dip/sauce,a squeezed of calamansi is a perfect addition to taste.