The Collaborative International Dictionary of English v.0.48:

Disappointment \Dis`ap*point"ment\, n. [Cf. F.
   d['e]sappointement.]
   1. The act of disappointing, or the state of being
      disappointed; defeat or failure of expectation or hope;
      miscarriage of design or plan; frustration.
      [1913 Webster]

            If we hope for things of which we have not
            thoroughly considered the value, our disappointment
            will be greater than our pleasure in the fruition of
            them.                                 --Addison.
      [1913 Webster]

            In disappointment thou canst bless.   --Keble.
      [1913 Webster]

   2. That which disappoints.

   Syn: Miscarriage; frustration; balk.
        [1913 Webster]

WordNet (r) 3.0 (2006):

disappointment
    n 1: a feeling of dissatisfaction that results when your
         expectations are not realized; "his hopes were so high he
         was doomed to disappointment" [syn: disappointment,
         letdown]
    2: an act (or failure to act) that disappoints someone [syn:
       disappointment, dashing hopes]

Moby Thesaurus II by Grady Ward, 1.0:

79 Moby Thesaurus words for "disappointment":
   Faustianism, awakening, ban, blackball, blackballing,
   blighted hope, blow, calamity, chagrin, cold comfort, complaint,
   dashed hopes, debunking, defeat, dejection, depression, dim view,
   disagreement, disapprobation, disapproval, disaster, discontent,
   discontentedness, discontentment, discouragement, disenchantment,
   disesteem, disfavor, disgruntlement, disillusion, disillusionment,
   displeasure, disrespect, dissatisfaction, dissatisfiedness,
   dissent, distaste, distress, divine discontent, enlightenment,
   envy, exclusion, failure, fiasco, fizzle, frustration,
   hope deferred, ill humor, indignation, low estimation, low opinion,
   mortification, objection, opposition, opposure, ostracism,
   peevishness, petulance, protest, querulousness, rebelliousness,
   regret, rejection, resentment, restiveness, restlessness,
   return to reality, rude awakening, sourness, sulkiness,
   thumbs-down, undeception, uneasiness, unfulfillment, unhappiness,
   unpleasure, unsatisfaction, unspelling, vexation of spirit