WordNet (r) 3.0 (2006):

pipe dream
    n 1: a fantastic but vain hope (from fantasies induced by the
         opium pipe); "I have this pipe dream about being emperor of
         the universe" [syn: pipe dream, dream]

Moby Thesaurus II by Grady Ward, 1.0:

73 Moby Thesaurus words for "pipe dream":
   Walter Mitty, absence of mind, absentmindedness, absorption,
   abstractedness, abstraction, airy hope, airy nothing, autism,
   bad dream, be absent, bemusement, brown study, bubble,
   castle-building, chimera, daydream, daydreamer, daydreaming,
   deception, deluded belief, delusion, depth of thought, dereism,
   divagate, dream, dream of, dream vision, dreaming, dreamland,
   dreamworld, engrossment, false belief, fantasy, fantasying,
   fit of abstraction, freak out, go woolgathering, golden dream,
   ignis fatuus, illusion, incubus, misbelief, misconception, moon,
   mooning, moonraking, muse, musefulness, musing, muted ecstasy,
   nightmare, pipe-dreaming, preoccupation, quixotic ideal, reverie,
   self-deceit, self-deception, self-delusion, stargaze, stargazing,
   stray, study, trance, trick, trip, unreal hope, utopia, vapor,
   vision, wander, woolgathering, wrong impression