After you screw something up, you magically and conveniently gain a vast amount of knowledge on what you SHOULD have done to avoid screwing up.
Although it generally occurs after you really messed up, it doesn't necessary always happen after such a negative situation.
Girl- "Hey Dave, I just tested positive on that pregnancy test, you dunce."
Dave- "You know, maybe I kind of, should have used a rubber at that party 2 weeks ago.... " <--(Hindsight)
Girl- "Yeah... that probably would have been a very good idea."
Recognition of the realities, possibilities of events, situations,decisions etc.. after they have occurred
In hindsight, we should have acted differently when Chris told us he was gay, now hes not talking to us.