One of the earliest constitutional protections we have–even before the Bill of Rights — is that neither the federal nor the states can pass ex post facto laws, which are laws that retroactively criminalize an act which was permitted before, or retroactively make the punishment worse than it was before, or changes the rules of evidence in such a way that makes it easier to get a conviction.

This means that if I do something that is not against the law today, and a law is passed forbidding it next week, I cannot be prosecuted for it, because the law criminalizing it was passed after the act. The term is derived from Latin meaning “out of the aftermath”.