Final answer:
Ex post facto laws are those that retroactively change the legal status of actions that were committed before the enactment of the law, which is prohibited in the U.S. for both federal and state legislation.
Step-by-step explanation:
Laws that inflict punishment for acts not illegal when they were committed are referred to as ex post facto laws. These laws have a retroactive effect, meaning they can either criminalize actions that were legal at the time they were undertaken or can increase the severity of punishment after the fact. This is in contrast to criminal statutes, which define conduct as illegal moving forward and are not applied retroactively. Common law and civil law represent different legal systems, neither of which should exercise ex post facto principles. It's fundamental in U.S. jurisprudence that both Congress and state legislatures are prohibited from enacting ex post facto laws, as they can lead to unfair and unjust outcomes for individuals.