A tuna is a large saltwater fish. The weight of a tuna varies enormously, from four pounds to more than 1,500 pounds!
It's common to refer to the kind of tuna that's sold as food in cans as "tuna fish." If you're mainly familiar with the tuna on your tuna fish sandwich, you might be surprised to learn that the Atlantic bluefin tuna lives to be about 50 years old, and can measure almost seven feet long. The word tuna comes from the Arabic root tun via the Spanish atún.