The Collaborative International Dictionary of English v.0.48:

Phantasmal \Phan*tas"mal\, a.
   Pertaining to, of the nature of, or resembling, a phantasm;
   spectral; illusive.
   [1913 Webster]

WordNet (r) 3.0 (2006):

phantasmal
    adj 1: resembling or characteristic of a phantom; "a ghostly
           face at the window"; "a phantasmal presence in the room";
           "spectral emanations"; "spiritual tappings at a seance"
           [syn: apparitional, ghostlike, ghostly,
           phantasmal, spectral, spiritual]

Moby Thesaurus II by Grady Ward, 1.0:

65 Moby Thesaurus words for "phantasmal":
   Barmecidal, Barmecide, aerial, air-built, airy, apparent,
   apparitional, astral, autistic, chimeric, cloud-born, cloud-built,
   cloud-woven, deceptive, delusional, delusionary, delusive,
   delusory, dereistic, dreamlike, dreamy, ectoplasmic, erroneous,
   ethereal, etheric, fallacious, false, fantastic, ghostish,
   ghostlike, ghostly, ghosty, illusional, illusionary, illusive,
   illusory, imaginary, incorporeal, misleading, ostensible,
   phantasmagoric, phantasmic, phantom, phantomic, phantomlike,
   psychic, seeming, self-deceptive, self-deluding, shadowy, specious,
   specterlike, spectral, spiritual, supernatural, supposititious,
   unactual, unfounded, unreal, unsubstantial, vaporous, vapory,
   visionary, wraithlike, wraithy