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]