The Animus is the machine that lets people relive genetic memories in Assassin's Creed.
The device is used by modern-day characters to access memories inherited through their DNA from historical ancestors. In the first game, Desmond Miles enters an Animus operated by Abstergo Industries and experiences the life of his ancestor Altaïr Ibn-La'Ahad.
The Animus is not itself a historical artifact from the period being explored. It is a modern interface that connects the present-day storyline to the playable historical sequences. The Helix is a later entertainment-oriented successor system, while the Bleeding Effect describes a consequence of prolonged Animus exposure rather than a machine.