The Apple of Eden is the Assassin's Creed artifact associated with controlling people's minds.
It is a Piece of Eden, a powerful device created by the Isu, the technologically advanced precursor civilization in the series' mythology. The Apple can project illusions, influence behavior, and manipulate perception, making it valuable to both Assassins and Templars.
The artifact appears in several games and historical storylines. In the first Assassin's Creed, Altaïr recovers an Apple from Robert de Sablé's forces and discovers its ability to affect crowds. Later games present other Apples and expand the lore surrounding their creation and use.
The broader term is Piece of Eden, because the series contains multiple Isu artifacts with different powers. The Apple is specifically the spherical device linked most strongly to mental influence; it should not be confused with the Staff of Eden or the Sword of Eden.