The Collaborative International Dictionary of English v.0.48:

Dreamer \Dream"er\ (dr[=e]m"[~e]r), n.
   1. One who dreams.
      [1913 Webster]

   2. A visionary; one lost in wild imaginations or vain schemes
      of some anticipated good; as, a political dreamer.
      [1913 Webster]

WordNet (r) 3.0 (2006):

dreamer
    n 1: someone who is dreaming
    2: someone guided more by ideals than by practical
       considerations [syn: idealist, dreamer]
    3: a person who escapes into a world of fantasy [syn:
       escapist, dreamer, wishful thinker]

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "dreamer":
   Don Quixote, Quixote, Utopian, daydreamer, dreamer of dreams,
   enthusiast, escapist, idealist, idealizer, illusionist,
   lotus-eater, prophet, rhapsodist, romancer, romantic, romanticist,
   seer, theorist, utopian, utopianist, utopianizer, visionary,
   wishful thinker