The Collaborative International Dictionary of English v.0.48:

Phantasm \Phan"tasm\, n. [L. phantasma. See Phantom, and cf.
   Fantasm.] [Spelt also fantasm.]
   1. An image formed by the mind, and supposed to be real or
      material; a shadowy or airy appearance; sometimes, an
      optical illusion; a phantom; a dream.
      [1913 Webster]

            They be but phantasms or apparitions. --Sir W.
                                                  Raleigh.
      [1913 Webster]

   2. A mental image or representation of a real object; a
      fancy; a notion. --Cudworth.
      [1913 Webster]

            Figures or little features, of which the description
            had produced in you no phantasm or expectation.
                                                  --Jer. Taylor.
      [1913 Webster]

WordNet (r) 3.0 (2006):

phantasm
    n 1: a ghostly appearing figure; "we were unprepared for the
         apparition that confronted us" [syn: apparition,
         phantom, phantasm, phantasma, fantasm, specter,
         spectre]
    2: something existing in perception only; "a ghostly apparition
       at midnight" [syn: apparition, phantom, phantasm,
       phantasma, fantasm, shadow]

Moby Thesaurus II by Grady Ward, 1.0:

124 Moby Thesaurus words for "phantasm":
   Masan, apparition, appearance, astral, astral spirit,
   bamboozlement, banshee, befooling, bluffing, brainchild, bubble,
   calculated deception, chimera, circumvention, conning, control,
   deceiving, deception, deceptiveness, defrauding, delirium,
   delusion, delusiveness, departed spirit, disembodied spirit,
   dupery, duppy, dybbuk, eidolon, enmeshment, ensnarement,
   entanglement, entrapment, fallaciousness, fallacy, falseness,
   fancy, fantasque, fantasy, fiction, figment, figure, flimflam,
   flimflammery, fond illusion, fooling, form, ghost, grateful dead,
   guide, hallucination, hant, haunt, hoodwinking, idle fancy, idolum,
   illusion, image, imagery, imagination, imagining, immateriality,
   incorporeal, incorporeal being, incorporeity, insubstantial image,
   invention, kidding, larva, lemures, maggot, make-believe, manes,
   materialization, mirage, myth, oni, outwitting, overreaching,
   phantasma, phantasmagoria, phantom, poltergeist, presence,
   putting on, revenant, romance, self-deception, shade, shadow,
   shape, shrouded spirit, sick fancy, snow job, song and dance,
   specter, spectral ghost, spirit, spoofery, spoofing, spook, sprite,
   subterfuge, swindling, theophany, thick-coming fancies, trickiness,
   tricking, trip, unsubstantiality, vapor, victimization, vision,
   waking dream, walking dead man, wandering soul, whim, whimsy,
   wildest dream, wildest dreams, willful misconception,
   wishful thinking, wraith, zombie