An Eidolon is a ghost, phantom or apparition. In Ancient Greek beliefs it was considered to be the surviving spirit of a dead person, transparent but visible and able to take on an apparently physical form identical to the dead person in the manner of a doppelganger. In Homer it is referred to in a more general sense as a phantom image of a person, perhaps even a person who is still alive.
The concept of the Eidolon was used in Ancient Greek literature many times and was also referenced by Edgar Allan Poe, H.P. Lovecraft and Clark Ashton Smith in their works. In gothic and weird horror fiction it was a common stock term, and used usually to refer to a specific phantom or spectral presence, for example in Poe the "Eidolon named Night" in his poem "Dream-Land".