What was the name of the artifact stolen by Altaïr in Assassin's Creed?

The artifact stolen by Altaïr in the original Assassin's Creed is the Apple of Eden.

Altaïr Ibn-La'Ahad and Robert de Sablé discover the Apple during events surrounding Solomon's Temple. The artifact is one of the Pieces of Eden, technologically advanced objects created by the First Civilization and later treated as legendary relics by human societies.

In the game, the Apple can influence human perception and behavior, which makes it valuable to both the Assassins and the Templars. Its powers also connect the historical storyline to the modern-day plot involving Desmond Miles and the Animus.

The series contains several different Pieces of Eden, including staffs, swords, and shrouds. Those objects belong to the same broad category but are not the artifact Altaïr steals in the original game. The Apple of Eden is the specific name required here.