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]

The Collaborative International Dictionary of English v.0.48:

Fantasm \Fan"tasm\, n. [See Phantasm, Fancy.]
   Same as Phantasm.
   [1913 Webster]

WordNet (r) 3.0 (2006):

fantasm
    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]