Etymology

Noun

1. An image or representation of an idea; a representation of an ideal form; an apparition of some actual or imaginary entity, or of some aspect of reality.
   • 1936, Henry Miller, Black Spring:

     As a species it is extinct; as an eidolon it retains its corporeality – but only if maintained in a state of equipoise.

   • 2006, Thomas Pynchon, Against the Day, Vintage 2007, p.697:

     Kit was sitting up staring into the dark at this eidolon, inelegantly turned out contrary to a whole raft of public-decency statutes, which had come monitory and breathing in to violate Kit's insomnia.

2. A phantom, a ghost or elusive entity.