The Apple of Eden is the famous Isu artifact repeatedly featured in Assassin's Creed games.
Apples of Eden are Pieces of Eden, technologically advanced objects created by the Isu, the ancient civilization that existed before humanity's recorded history in the series' mythology. They can influence minds and produce other extraordinary effects, which makes them highly valuable to both Assassins and Templars.
The artifact is not a single ordinary apple and should not be confused with every Piece of Eden. The franchise also includes objects such as swords, staffs, shrouds, and crystal spheres. The Apple became especially recognizable because it appears in the original game and in several later stories, often as the focus of a struggle between the two rival factions.