The Animus is the Assassin's Creed device that lets users experience ancestral memories.
In the series' modern-day storyline, the Animus uses a person's genetic memories to reconstruct experiences inherited from ancestors. A user enters the machine and interacts with a simulated historical world, allowing the games to move between a contemporary framing story and historical settings.
The device explains why characters such as Desmond Miles can experience the lives of people including Altaïr Ibn-La'Ahad and Ezio Auditore. It also provides the series with a consistent way to connect different historical protagonists without claiming that the modern user literally travels through time.
The Animus is often confused with Pieces of Eden, but the two have different functions. Pieces of Eden are powerful artifacts linked to the Isu, while the Animus is a human-made machine. The Bleeding Effect is another related term: it describes skills or memories that can persist in a user after prolonged exposure to an ancestor's experiences.