The Apple of Eden is the First Civilization artifact that often appears as a glowing sphere in Assassin's Creed.
The Apples are Pieces of Eden created by the Isu, the technologically advanced civilization that existed before humanity's recorded history. In the games, an Apple can influence minds, create illusions, and interact with other advanced artifacts. Its abilities help explain why Assassins and Templars repeatedly seek control of it.
The artifact is inspired in part by the biblical image of an apple, but Assassin's Creed gives it a science-fiction explanation rather than treating it as a religious object. Multiple Apples exist across the series, and their effects vary by story and game.
The Staff of Hermes, Sword of Eden, and Shroud of Eden are also Pieces of Eden, but they have different forms and functions. “Apple of Eden” therefore identifies the spherical artifact specifically, not every First Civilization relic.