The fictional ancient artifact that drives the plot of Assassin's Creed II is the Apple of Eden.
The Apple is one of the Pieces of Eden, powerful artifacts created by the Isu, the technologically advanced beings who preceded humanity in the series' mythology. It can influence minds and project visions, making it valuable to both Assassins and Templars.
In Assassin's Creed II, Ezio encounters the Apple during his search for the people responsible for his family's destruction. The artifact also links the Renaissance storyline to the modern-day conflict involving Desmond Miles and Abstergo.
The Apple is not a literal ordinary apple, despite its name and familiar shape. Other Pieces of Eden include swords, staffs, and shrouds, so “Piece of Eden” is the category, while “Apple of Eden” identifies this specific artifact.