The Animus is the machine that lets Desmond relive his ancestors' memories in Assassin's Creed.
In the series' modern-day storyline, the Animus reads genetic memories preserved in a person's DNA. Desmond Miles uses it to experience the lives of ancestors such as Altaïr Ibn-La'Ahad and Ezio Auditore, allowing the player to enter historical settings while the modern plot continues around him.
The Animus is fictional technology and serves as the franchise's narrative bridge between present-day characters and historical adventures. It is not the same as Eagle Vision, which is a perception ability used by Assassins and other characters. Helix is another fictional entertainment and memory-related system introduced in later games, while the Observatory is a location and device associated with Black Flag's plot. These terms are related to the universe but are not synonyms for the Animus.