The Apple-shaped First Civilization artifact in Assassin’s Creed is called an Apple of Eden.
Apples of Eden are Pieces of Eden, technologically advanced artifacts created by the First Civilization, also known as the Isu. Their powers can influence perception, control minds, and generate other effects that later cultures interpret as supernatural.
The series uses these artifacts to connect its historical stories with its modern science-fiction plot. Characters from different eras seek, protect, or exploit Apples because they can shift the balance between Assassins and Templars.
An Apple of Eden is not the only Piece of Eden. Other artifacts include swords, staffs, shrouds, and the Trident of Eden. The apple shape is the specific visual form associated with this artifact category, rather than a generic name for every Piece of Eden.