Source : Webster's Revised Unabridged Dictionary (1913)

Evanescence \Ev`a*nes"cence\, n.
   The act or state of vanishing away; disappearance; as, the
   evanescence of vapor, of a dream, of earthly plants or hopes.
   --Rambler.

Source : WordNet®

evanescence
     n : the event of fading and gradually vanishing from sight; "the
         evanescence of the morning mist"