The different parts of a tuna can be broadly categorized into two: the fatty portion called "toro" and the other parts known as "akami" or red meat. Toro can also be further divided into various sections, but the most fatty portion known as "otoro" (大トロ) and the representative toro section with a moderate amount of fat known as "chutoro" (中トロ) are often mentioned. Generally, otoro refers to the belly area closer to the head of the tuna, while chutoro refers to the meat found in the belly and back areas. However, there is no clear definition that distinguishes otoro from chutoro. Recently, Sushineta, which is more than twice the price of akami, has gained popularity. However, in old Japan, akami was the preferred part of the tuna, while toro was considered a discarded section. The people of old Japan preferred a clean taste over a fatty one, and due to the lack of refrigeration and preservation techniques, the fatty toro section easily underwent flavor changes. During the Edo period, it was even considered a fish that not even a cat would bother to touch, and was called "nekomatagi" (猫またぎ) meaning "a fish that a cat simply passes by." It was only later, as the Japanese palate evolved and refrigeration technology advanced, that toro gradually gained its current popularity. The term "toro" originates from the Japanese word "torokeru" (とろける), meaning to melt. It was in 1918 at the Nopo sushi restaurant "Yoshinozushi" in Tokyo that the term "toro" began to be used and spread nationwide. Prior to that, it was referred to as "abu" (アブ), derived from the first character of "abura" (脂), meaning fat, indicating the fatty section.

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.