The Collaborative International Dictionary of English v.0.48:

Disillusionment \Dis`il*lu"sion*ment\, n.
   The act of freeing from an illusion, or the state of being
   freed therefrom.
   [1913 Webster]

WordNet (r) 3.0 (2006):

disillusionment
    n 1: freeing from false belief or illusions [syn:
         disenchantment, disillusion, disillusionment]