Source : Webster's Revised Unabridged Dictionary (1913)

Daydream \Day"dream`\ (-dr[=e]m`), n.
   A vain fancy speculation; a reverie; a castle in the air;
   unfounded hope.

         Mrs. Lambert's little daydream was over. --Thackeray.

Source : WordNet®

daydream
     n : absent-minded dreaming while awake [syn: {reverie}, {revery},
          {daydreaming}, {oneirism}, {air castle}, {castle in the
         air}, {castle in Spain}]
     v 1: have a daydream; indulge in a fantasy [syn: {dream}, {woolgather},
           {stargaze}]
     2: have dreamlike musings or fantasies while awake; "She looked
        out the window, daydreaming" [syn: {moon}]