Toro' is fish meat cut from a specific part of tuna, and is used as an ingredient for sushi, etc. It refers to tuna meat cut from the ventral part, and contains high fat. The word 'Toro' is derived from the texture of tuna meat, which melts in the mouth ('toro-ri'). Prior to the establishment of the word 'Toro,' the meat in question was called 'Abu' because of its fatty quality ('abura-mi'). In older times, particularly prior to the Edo period, tuna meat was represented by lean meat, whereas Toro, which goes sour more quickly than lean meat, was regarded as an insignificant material and discarded, or, if any, tasted as heated meat in a hot pot featuring tuna meat and spring onions (Negima Nabe). It was said that even cats avoided Toro. These days, however, due to the wide acceptance of animal fat as delicious food and improvement in preservation and transportation technologies, fresh and delicious Toro has become available, and people consider Toro as luxury food. Also, prices have skyrocketed as the modern era arrived, with Toro selling at more than double the value of lean meat currently.
In particular, the part that contains higher fat is called 'O-toro' while the part with slightly less fat is called 'Chu-toro.'
Parts other than O-toro and Chu-toro are called 'lean meat' or simply 'tuna meat' and treated as something different from Toro. In general, ventral meat more than dorsal meat, anterior meat more than posterior meat, and exterior meat more than interior meat contains higher fat. In principle, 'O-toro' and 'Chu-toro' refers to the anterior and posterior part of ventral meat, respectively. Recently, due to the establishment of complete aquaculture technology, hybrid tuna, whose whole body consists of Toro, have also been created.
Sometimes, however, fatty meat other than tuna meat is also called 'Toro.'
For example, fatty bonito sashimi is called 'Toro bonito.'
In Hokkaido, there is a local dish called 'Gyu-toro don' (rice with raw beef) that uses raw beef as an ingredient. As for pork, the part called 'Ton-toro' (Toro pork) is available. As the word 'Toro' is not clearly defined in terms of types of meat, sellers use the word 'Toro' as a tool of sales promotion strategies because Toro tuna meat has the image of luxury food.