The Collaborative International Dictionary of English v.0.48:

Eidolon \Ei*do"lon\, n. [NL., fr. Gr. ? image. See Idol.]
   An image or representation; a form; a phantom; an apparition.
   --Sir W. Scott.
   [1913 Webster]

Moby Thesaurus II by Grady Ward, 1.0:

114 Moby Thesaurus words for "eidolon":
   Masan, angle, apparition, appearance, aspect, astral,
   astral spirit, banshee, brainchild, bubble, chimera, configuration,
   control, delirium, departed spirit, disembodied spirit, duppy,
   dybbuk, effect, facet, fancy, fantasque, fantasy, fashion, feature,
   fiction, figment, figure, form, gestalt, ghost, grateful dead,
   guide, guise, hallucination, hant, haunt, idle fancy, idolum,
   illusion, image, imagery, imagination, imagining, imago,
   immateriality, impression, incorporeal, incorporeal being,
   incorporeity, insubstantial image, invention, larva, lemures,
   light, likeness, lineaments, look, maggot, make-believe, manes,
   manner, materialization, myth, oni, phantasm, phantasma,
   phantasmagoria, phantom, phase, phasis, poltergeist, presence,
   reference, regard, respect, revenant, romance, seeming, semblance,
   shade, shadow, shape, shrouded spirit, sick fancy, side,
   simulacrum, slant, specter, spectral ghost, spirit, spook, sprite,
   style, theophany, thick-coming fancies, total effect, trip, twist,
   unsubstantiality, vapor, view, viewpoint, vision, waking dream,
   walking dead man, wandering soul, whim, whimsy, wildest dream,
   wildest dreams, wise, wraith, zombie