A bonito is a medium-sized fish that's a close relation of the tuna. When smoked and fermented, bonito is used to flavor many Japanese soups.
Bonitos are in the same family as tuna and mackerel, and they're sometimes used as a less expensive substitute for skipjack tuna. In Japanese cuisine, the fish is commonly used in the form of "bonito flakes," or katsuobushi. These intensely flavorful dried flakes are added to dashi broth, which is the base of most miso soup. Bonito is a Spanish word that means "the good one."