The Animus is the device that allows characters to relive genetic memories in Assassin's Creed.
In the series, the Animus reads inherited genetic information and presents a user's ancestors' experiences as an interactive simulation. Desmond Miles first uses an Animus under Abstergo's control, allowing him to experience the life of Altaïr Ibn-La'Ahad in the first game.
The device provides the franchise's central link between modern-day characters and historical protagonists. It also explains why a contemporary character can explore events in many different centuries without the story requiring literal time travel.
The Animus is not the same as an Apple of Eden. Apples are powerful Pieces of Eden associated with the ancient Isu, while the Animus is a modern machine based on genetic-memory research. “Bleeding Effect” is another related term: it describes abilities or memories that can carry over from prolonged Animus exposure, rather than the device itself.