In Assassin's Creed, the Animus is the memory-replaying machine used by Desmond Miles.
The Animus allows a modern person to access genetic memories inherited from ancestors. In the first game, Abstergo Industries places Desmond in the machine so he can relive the experiences of Altaïr Ibn-La'Ahad, an Assassin living during the Third Crusade. The device gives the series its central structure: a modern storyline connected to historical adventures.
The Animus does not literally send Desmond through time. Instead, it reconstructs ancestral memories through genetic information, with the user experiencing them as an interactive simulation. Later games show several Animus models and related interfaces, including systems used by other characters and organizations.
A common mix-up is calling the Apple of Eden the machine. The Apple is a Piece of Eden, an ancient technological artifact within the story, while the Animus is the modern device that accesses memories. Eagle Vision is a special perception ability, and the Bleeding Effect is a consequence of prolonged Animus exposure.