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]