In Assassin's Creed, the Animus is the machine that lets users experience ancestral memories.
The Animus is a fictional device developed by Abstergo Industries. It reads genetic memories inherited through a person's ancestry and presents them as interactive experiences. Desmond Miles uses the machine to relive memories connected to earlier Assassins, including Altaïr Ibn-La'Ahad and Ezio Auditore.
The device provides the series with a narrative link between historical adventures and a modern-day storyline. It also explains why a contemporary character can appear in several different historical periods without the story treating those periods as ordinary time travel.
The Helix is a later Abstergo entertainment platform based on related technology, so the two names are sometimes mixed up. Eagle Vision is instead an Assassin ability that highlights people, objects, and points of interest; it is not a machine.