The artifact frequently called the Apple of Eden is a Piece of Eden in Assassin's Creed. Pieces of Eden are advanced objects created by the Isu, an ancient civilization that existed before recorded human history in the series' fiction.
The Apple is usually depicted as a spherical artifact capable of influencing minds, generating illusions, and interacting with technology. Its powers vary by story and individual Apple, so it is better understood as one type of Piece of Eden rather than the name for every artifact in the series.
Other Pieces of Eden include the Staff, Swords, Shrouds, and various crystal artifacts. The term Apple of Eden is therefore a specific popular name, while Piece of Eden is the broader canonical category. The artifact is central to conflicts between the Assassins and Templars across multiple games.