Ponzu

Ponzu is a citrus-based sauce commonly used in Japanese cuisine. It is tart, with a thin, watery consistency. Ponzu shōyu or ponzu jōyu (ポン酢醤油) is ponzu with soy sauce (shōyu) added, and the mixed dark brown product is widely referred to as simply ponzu. The term originally came into the Japanese language as ponsu as a borrowing of the now obsolete Dutch word pons, meaning punch as in a beverage made from fruit juices. The sour nature of this sauce led to the final -su being written with the character su (酢), meaning "vinegar". Ponzu is made by simmering mirin, rice vinegar, katsuobushi flakes (from tuna), and seaweed (kombu) over medium heat. The liquid is then cooled, strained to remove the katsuobushi flakes, and finally the juice of one or more of the following citrus fruits is added: yuzu, sudachi, daidai, kabosu, or lemon.

Source: Wikipedia