Upon/əˈpɑːn/ Definition 1 of 4Pro Tip 1/2Story Beginnings Used to start fairy tales, implying a timeless, magical setting. Once upon a time, in a land far away, there was a talking frog. Pro Tip 2/2Idiomatic Expressions Often found in set expressions that signal something unexpected or accidental. I happened upon an old friend at the market today.