Source : Webster's Revised Unabridged Dictionary (1913)

Wistful \Wist"ful\, a. [For wishful; perhaps influenced by
   wistly, which is probably corrupted from OE. wisly certainly
   (from Icel. viss certain, akin to E. wit). See {Wish}.]
   1. Longing; wishful; desirous.

            Lifting up one of my sashes, I cast many a wistful,
            melancholy look towards the sea.      --Swift.

   2. Full of thought; eagerly attentive; meditative; musing;
      pensive; contemplative.

            That he who there at such an hour hath been, Will
            wistful linger on that hallowed spot. --Byron.
      -- {Wist"ful*ly}, adv. -- {Wist"ful*ness}, n.

Source : WordNet®

wistfulness
     n : a sadly pensive longing