The Collaborative International Dictionary of English v.0.48:

Phantom \Phan"tom\, n. [OE. fantome, fantosme, fantesme, OF.
   fant[^o]me, fr. L. phantasma, Gr. fa`ntasma, fr. fai`nein to
   show. See Fancy, and cf. Pha["e]ton, Phantasm,
   Phase.]
   That which has only an apparent existence; an apparition; a
   specter; a phantasm; a sprite; an airy spirit; an ideal
   image.
   [1913 Webster]

         Strange phantoms rising as the mists arise. --Pope.
   [1913 Webster]

         She was a phantom of delight.            --Wordsworth.
   [1913 Webster]

   Phantom ship. See Flying Dutchman, under Flying.

   Phantom tumor (Med.), a swelling, especially of the
      abdomen, due to muscular spasm, accumulation of flatus,
      etc., simulating an actual tumor in appearance, but
      disappearing upon the administration of an an[ae]sthetic.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Phantom \Phan"tom\, a.
   Being, or of the nature of, a phantom.

         Phantom isles are floating in the skies. --B. Taylor.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

phantom
    adj 1: something apparently sensed but having no physical
           reality; "seemed to hear faint phantom bells"; "the
           amputee's illusion of a phantom limb"
    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:

200 Moby Thesaurus words for "phantom":
   Barmecidal, Barmecide, Dracula, Frankenstein, Masan, Wolf-man, air,
   airy, airy nothing, apparent, apparition, apparitional, appearance,
   asomatous, astral, astral spirit, autistic, banshee, bodiless,
   bogey, bogeyman, brainchild, bubble, bugaboo, bugbear, chimaera,
   chimera, chimeric, control, decarnate, decarnated, deceptive,
   delirium, delusion, delusional, delusionary, delusive, delusory,
   departed spirit, dereistic, discarnate, disembodied,
   disembodied spirit, dreamlike, dreamy, duppy, dybbuk, ectoplasmic,
   eidolon, erroneous, ether, ethereal, etheric, extramundane,
   fallacious, false, false image, fancy, fantasque, fantastic,
   fantasy, fee-faw-fum, fiction, figment, figure, form, frightener,
   ghost, ghostish, ghostlike, ghostly, ghosty, ghoul, grateful dead,
   guide, hallucination, hant, haunt, hobgoblin, holy terror, horror,
   idle fancy, idolum, illusion, illusional, illusionary, illusive,
   illusory, image, imagery, imaginary, imagination, imagining,
   immaterial, immateriality, impalpable, imponderable, incorporate,
   incorporeal, incorporeal being, incorporeity, incubus,
   insubstantial, insubstantial image, intangible, invention, larva,
   lemures, maggot, make-believe, manes, materialization, mirage,
   misleading, mist, monster, myth, nightmare, nonmaterial,
   nonphysical, occult, ogre, ogress, oni, ostensible, otherworldly,
   phantasm, phantasma, phantasmagoria, phantasmagoric, phantasmal,
   phantasmic, phantomic, phantomlike, phasm, phenomenon, poltergeist,
   presence, psychic, revenant, romance, scarebabe, scarecrow, scarer,
   seeming, self-deceptive, self-deluding, shade, shadow, shadowy,
   shape, shrouded spirit, sick fancy, smoke, specious, specter,
   specterlike, spectral, spectral ghost, spectrum, spirit, spiritual,
   spook, sprite, succubus, supernatural, supposititious, terror,
   theophany, thick-coming fancies, thin air, transmundane, trip,
   unactual, unearthly, unembodied, unextended, unfleshly, unfounded,
   unphysical, unreal, unsubstantial, unsubstantiality, unworldly,
   vampire, vapor, vision, visionary, waking dream, walking dead man,
   wandering soul, werewolf, whim, whimsy, wildest dream,
   wildest dreams, wraith, wraithlike, wraithy, zombie