The Japanese dried, fermented, and smoked bonito flakes used to make dashi broth are called katsuobushi.
Katsuobushi is traditionally made from skipjack tuna, Katsuwonus pelamis, which is sometimes broadly called bonito. Fillets are simmered, smoked repeatedly, dried, and, in the most traditional process, treated with mold cultures and sun-dried. The result is an exceptionally hard block that can be shaved into delicate flakes. Its concentrated savory flavor comes partly from inosinate, a compound associated with umami.
The flakes are a major dashi ingredient, often paired with kombu, an edible kelp rich in glutamic acid. Together, the ingredients create a stronger savory effect than either contributes alone. Katsuobushi is also sprinkled over dishes such as okonomiyaki and takoyaki, mixed into rice seasonings, or combined with soy sauce as okaka.
A common mix-up is Hondashi, which is a commercial instant dashi product rather than the traditional fish ingredient itself. Niboshi refers to dried small fish, especially sardines or anchovies, while kombu is kelp. Thin hanakatsuo flakes are commonly used as a topping; thicker shavings are preferred for stock-making.