The Collaborative International Dictionary of English v.0.48:

hindsight \hindsight\ n.
   understanding the nature of an event after it has happened;
   as, hindsight is always clearer than foresight.
   [WordNet 1.5]

   20-20 hindsight, twenty-twenty hindsight Perfect
      understanding of an event after it has happened; -- a term
      usually used with sarcasm in response to criticism of
      one's decision, implying that the critic is unfairly
      judging the wisdom of the decision in light of information
      that was not available when the decision was made.
      [PJC]

WordNet (r) 3.0 (2006):

hindsight
    n 1: understanding the nature of an event after it has happened;
         "hindsight is always better than foresight"

Moby Thesaurus II by Grady Ward, 1.0:

23 Moby Thesaurus words for "hindsight":
   commitment to memory, exercise of memory, flashback,
   learning by heart, looking back, memoir, memorization, memorizing,
   recall, recalling, recollecting, recollection, reconsideration,
   reflection, remembering, remembrance, reminiscence, retrospect,
   retrospection, review, rote, rote memory, study