The Animus is the fictional device that lets modern users experience their ancestors' genetic memories in Assassin's Creed.
The machine is used by Abstergo Industries to access inherited memories stored in a person's DNA. In the first game, Desmond Miles is placed in the Animus so researchers can explore the memories of his ancestor Altaïr Ibn-La'Ahad. Later stories use increasingly advanced versions of the technology and introduce other users.
The Animus provides the narrative bridge between modern-day characters and historical settings. It also explains why players can move between eras and control different ancestors while the modern plot continues outside the simulations.
The Apple of Eden is a separate Isu artifact, not a memory-viewing machine. The Bleeding Effect is a condition caused by prolonged Animus exposure, and the Eye of Horus is not the series' name for this device.