Definition
'Ex post facto' is a Latin term that means 'from after the fact.' It refers to a law that retroactively changes the legal consequences of actions that were committed, or relationships that existed, before the enactment of the law. Basically, you can’t be punished for something that wasn't illegal when you did it. It's a principle that protects people from being unfairly penalized for past actions. Think of it as a legal time machine that doesn't work.