Citrus sudachi

Sudachi (Citrus sudachi; Japanese: スダチ or 酢橘) is a small, round, green citrus fruit of Japanese origin that is a specialty of Tokushima Prefecture in Japan. Harvested before it fully ripens to yellow, it is tart and not eaten as a table fruit but used to flavor sauces and marinades, desserts, and drinks in place of lemon or lime. Genetic analysis shows it to be the product of a cross between a yuzu and another citrus fruit akin to the koji and tachibana orange....read more on Wikipedia.