Definition of "tuna"

tuna¹

/ˈtjuː.nə/

noun
countable and uncountable, plural tuna or tunas
1. Any of several species of fish of the genus Thunnus in the family Scombridae.
  Quotations
  Tuna was carried down by the flood; and when Maui saw him in the net he stretched forth his arm and with a blow of his stone axe smote Tuna and cut off his head, and it and the tail fell into the ocean... The head became fish, and the tail became the koiro (ngoiro—conger-eel).
  1887, John White, The Ancient History of the Maori, page 84
2. The edible flesh of the tuna.

tuna²

/ˈtjuː.nə/

noun
plural tunas
1. The prickly pear, a type of cactus native to Mexico in the genus Opuntia.
2. The fruit of the cactus.