The Collaborative International Dictionary of English v.0.48:

Visualize \Vis"u*al*ize\, v. t.
   1. To make visual, or visible. [Written also visualise.]
      [1913 Webster]

   2. to see in the imagination; to form a mental image of.
      [1913 Webster +PJC]

            No one who has not seen them [glaciers] can possibly
            visualize them.                       --Lubbock.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Visualize \Vis"u*al*ize\, v. i.
   To form a mental image of something not present before the
   eye at the time.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

visualize
    v 1: imagine; conceive of; see in one's mind; "I can't see him
         on horseback!"; "I can see what will happen"; "I can see a
         risk in this strategy" [syn: visualize, visualise,
         envision, project, fancy, see, figure, picture,
         image]
    2: view the outline of by means of an X-ray; "The radiologist
       can visualize the cancerous liver" [syn: visualize,
       visualise]
    3: form a mental picture of something that is invisible or
       abstract; "Mathematicians often visualize" [syn: visualize,
       visualise]
    4: make visible; "With this machine, ultrasound can be
       visualized" [syn: visualize, visualise]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "visualize":
   anticipate, apprehend, bring to mind, call to mind, call up,
   conceive, conjure up, contemplate, divine, envisage, envision,
   fancy, feature, foreknow, image, imagine, just see, look upon,
   objectify, picture, prevision, realize, regard, represent, see,
   summon up, view, vision