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]