An eyeball is a SLASH'EM comestible that serves as the base item for the Eye of the Beholder. It has strong parallels with a severed hand. It is not randomly generated, but you can find a normal one in a bones level if you already have the Eye of the Beholder, or you can even wish for one.
Since eating any artifact eyeball does 150-199 damage, the game will always ask for confirmation upon attempting to eat one("Are you sure you want to eat that? [ynq] (n)"). However, the only artifact eyeball in the game is the Eye of the Beholder. When eaten, an eyeball gives 10 nutrition points with the message: "This eyeball is delicious!". In contrast to the Eye of the Beholder, an eyeball can't be sacrificed.