Real Dictionary

tuna
variable noun: Tuna or tuna fish are large fish that live in warm seas and are caught for food.

tuna in American English 1
any of various, usually large, marine, scombroid, food and game fishes (esp. genus Thunnus), including the albacore
noun: any of several large food and game fishes of the family Scombridae, inhabiting temperate and tropical seas

tuna in American English 2
any of various prickly pears (esp. Opuntia tuna), cultivated for their edible fruits
noun: any of various prickly pears, esp. either of two erect, treelike species, Opuntia tuna or O. ficus-indica, of Mexico, bearing a sweet, edible fruit

tuna in British English 1
noun: any of various large marine spiny-finned fishes of the genus Thunnus, esp T. thynnus, chiefly of warm waters: family Scombridae. They have a spindle-shaped body and widely forked tail, and are important food fishes

tuna in British English 2
noun: any of various tropical American prickly pear cacti, esp Opuntia tuna, that are cultivated for their sweet edible fruits