This fruit, scientifically referred to as Citrofortunella mitis (Citrus reticulata x Fortunella sp), i.e. a hybrid of a Mandarin orange and the Kumquat, is an essential ingredient in Philippine cooking (according to the Oxford Companion to Food). It is a small, flavor and acid packed fruit that has a distinct citrus flavor, sometimes described as a cross between a lemon and a mandarin orange. It is used in kinilaw (raw fish marinated in vinegar and kalamansi), to marinate meats, to squeeze over broiled food, noodles or fruit, to make juice, or candied into something sweet and sour at the same time.
This lime is indigenous to the country, or at least it is now, though its ancestors may have come from Mainland China. It grows readily throughout the archipelago and bears fruit all year long. They seem to be more abundant towards the summer but that may just be my imagination. At the market recently, they could be had for P40 a kilo for fresh, large and bright specimens. I bought several kilos that I made into juice or just used over fruit and in my favorite beefsteak tagalog recipe dripping with onions.
When purchasing, look for dark green, bright, glossy fruit that are firm to the touch. I prefer the somewhat larger ones and as with other citrus, fresh leaves still attached to the fruit are a good sign that the shipment you are picking from has just arrived. They store easily in the fridge and should last 5-7 days. The flavor is considered so unique it now features in exotic ice creams/gelato in Paris and is being commercially developed as a flavoring by some food companies based in Southeast Asia.