Hindsight is the ability to understand a situation only after it has already happened. At the time you may not see the danger, the kind choice, or the better plan; later, looking back, it feels obvious. People say in hindsight to mean “now that I look back,” and they say hindsight is 20/20 to mean looking back makes everything seem sharp, like perfect eyesight. You cannot have hindsight about tomorrow — it only works after the fact. It is ordinary, slightly thoughtful English: not slang, not stiff. You will hear it when someone is a little sorry, or a little wiser, about a past choice.