Toro (toh-roh) is the term for the fatty part of the tuna, found in the belly portion of the fish. Toro is further broken up into two distinct subtypes, and they are more expensive due to their relative scarcity as a proportion of the entire fish.
Chutoro (choo-toh-roh), which is sometimes labeled chu-toro, is the belly area of the tuna along the side of the fish between the akami and the otoro. It is often preferred because it is fatty but not as fatty as otoro.
Otoro (oh-toh-roh), which is sometimes labeled o-toro, is the fattiest portion of the tuna, found on the very underside of the fish. This cut is fatty almost to the point of falling apart and can literally melt in your mouth.