A large tuna distributed in the tropical and temperate waters of the Pacific Ocean, including the coasts of Japan. The standard name in Japanese is "Kuromaguro" (黒鮪), but it is generally distributed under the name "Honmaguro" (本鮪). Typically, when fully grown, it can reach 3 meters in length and weigh up to 300 kilograms. Among tunas, the bluefin tuna is the largest and most expensive high-grade species, with prices reaching tens of millions of yen per fish. In Japan, bluefin tuna caught using the "Ipponzuri" (一本釣り, single-hook fishing) method is considered the highest quality. There are several theories about the origin of the name "maguro" (tuna). One theory is that it is called "maguro" (眼黒) because of its black eyes. Another theory is that because its back is black, it looks like a small black mountain from the ship, hence "maguro" (真黒). Another theory is that it turns black when left at room temperature, hence the name maguro.

Tuna parts can mainly be divided into two: the fatty section known as Toro and the other part called Akami (red flesh). Although Toro can be further subdivided, the highest fat content is in Otoro (Tuna belly) and Akami, and Chutoro (Tuna mid-belly) which is a mix of Akami and fat, can be considered as the representative parts of Toro. Typically, Otoro refers to the belly near the head of the tuna, while Chutoro refers to the meat on the tuna's back and abdomen, but in reality, there's no clear distinction between Otoro and Chutoro. Recently, Akami is priced over twice that of Chutoro, making it a popular sushi ingredient. However, in ancient Japan, people consumed Akami, and Toro was discarded. In ancient Japan, people preferred a cleaner taste, and because refrigeration and distribution technologies were not developed, the fatty Toro easily deteriorated in taste. During the Edo period, it was even called a fish that even a cat wouldn't touch, implying that cats would simply pass it by. As the Japanese palate evolved and refrigeration technology developed, Toro gradually gained its current popularity. The term "Toro" originates from the Japanese word "とろける", meaning "melt". In 1918, the old sushi shop "Yoshinozushi" in Tokyo began calling it "Toro", which then spread and settled nationwide. Before that, because it was a fatty part, it was called "Abu" (oil), taken from the term "脂" for fat.