The Animus is the technology used to relive ancestral memories in Assassin’s Creed.
In the series, the Animus is a machine that lets a person access genetic memories inherited from ancestors. The user’s mind experiences reconstructed historical events, creating the main link between the modern-day storyline and the playable historical settings. Desmond Miles is the first major modern protagonist to use it extensively.
The device is associated with Abstergo Industries, a modern corporation secretly controlled by the Templar Order. The Assassin Brotherhood uses related technology as well, especially in later games. The Animus therefore functions both as a narrative explanation for historical gameplay and as an important object in the modern conflict.
The Helix is a separate entertainment platform and memory-experiencing system seen in later stories. The Apple of Eden is a Piece of Eden, not a machine, while an Observatory is a specific location and device connected to Black Flag.