The Animus is the device that lets characters relive ancestral memories in Assassin's Creed.
In the series, the Animus is a machine that accesses genetic memories inherited from a person's ancestors. Modern-day subjects use it to experience historical events through the perspective of an ancestor, allowing the games to move between contemporary laboratories and past civilizations.
Desmond Miles uses an early Animus under the direction of Abstergo Industries in the first games. Later stories introduce different versions, including portable and entertainment-oriented systems. The fictional technology explains why a modern character can participate in events involving Altaïr, Ezio, or other historical Assassins.
The Animus is not the same thing as an Apple of Eden. Apples are powerful Pieces of Eden with mind-control and other abilities, while the Animus is a machine. The Bleeding Effect is a separate consequence: repeated exposure can cause a user to acquire skills or memories associated with an ancestor.