The virtual reality system used by modern characters in Assassin's Creed is the Animus.
The Animus allows a user to access genetic memories inherited from ancestors. In the series, this technology lets modern characters experience historical lives while remaining in the present. The device frames the historical adventures and connects otherwise separate protagonists, locations, and time periods.
Abstergo Industries develops and uses Animus technology, while the Assassin characters also adapt it for their own purposes. Helix is a later entertainment platform based on related technology, not the general name for the device. Eagle Vision is an Assassin ability that highlights people and objects in the game world, so it is also distinct from the Animus.