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1.
Phrase used to describe the fact that it is easy for one to be knowledgable about an event after it has happened.

IE: An individual has a realization about the event that should have been obvious all along, yet they didn't catch on because they were acting in the heat of the moment.

Also allows one to learn from their mistakes.
After looking back on the situation, Jim realized that quitting his job was a foolish act.

Jim: "Dad, why did you let me quit my job without interceding?"

Dad: "Hindsight is 20/20, son, learn from your mistakes."
by TheWiseJanitor July 29, 2006