(Also sp. "kalamansi")
Other names may include Calamunding, Calamondin, Citrus mitis Blanco, etc.
The calamansi is a citrus fruit from Southeast Asia, sometimes known as "the lemon of the tropics" or "Philippine lemon", though personally I find the taste to actually be more akin to that of a lime, with a fruitiness perhaps somewhat recalling pineapple juice. (Most noticeable when the juice is sweetened with sugar or honey and made into a beverage)
Quite popular in Singapore, where it is readily available from hawker stands, as pure juice or as frozen concentrate in most grocery stores there. (where I first tried it) Or in the United States it can usually be found at Philippino and other Southeast Asian grocers, as well as many of the Asian chain grocers such as 99 Ranch in the San Francisco Bay Area. (And elsewhere, I'm sure)