The Animus is the device that lets modern people relive genetic memories in Assassin's Creed.
The series explains that memories are inherited through genetic material, but they cannot normally be experienced directly. The Animus interprets those memories and presents them as an interactive simulation, allowing a user such as Desmond Miles to experience the lives of ancestors.
The device provides the narrative link between modern-day scenes and historical settings. It also explains why players can revisit memories, synchronize actions, and sometimes encounter glitches or changes in the simulation.
Helix is the name of a later entertainment and research platform associated with Abstergo, not the original generic device. Eagle Vision is an Assassin ability, while the Bleeding Effect describes a consequence of prolonged Animus exposure rather than the machine itself.