What is an ex post facto law?

An ex post facto law is specifically defined as a law that retroactively changes the legal consequences or status of actions that were committed before the enactment of the law. This means it punishes actions that were legal at the time they were performed or increases penalties for acts that occurred prior to the law being enacted.

This concept is significant in United States law and is specifically prohibited by the Constitution in Article I, Section 9, which safeguards individuals from being punished for actions that were not illegal at the time they were committed. The other choices focus on different types of legislation. For example, a law that applies to future actions would be the opposite of an ex post facto law, while laws granting immunity and those requiring public notice do not pertain to the retroactive application of legal penalties.