Katsuobushi is simmered, smoked and fermented skipjack tuna. It is also known as bonito flakes or broadly as okaka (おかか).

The skipjack tuna is a perciform fish in the tuna family, Scombridae, and is the only member of the genus Katsuwonus. It is also known as katsuo, arctic bonito, mushmouth, oceanic bonito, striped tuna or victor fish. It grows up to 1 m (3 ft) in length. It is a cosmopolitan pelagic fish found in tropical and warm-temperate waters. It is a very important species for fisheries.