What defines an ex post facto law?

An ex post facto law is fundamentally defined by its retrospective application, meaning it punishes individuals for acts that were not considered illegal at the time they were committed. This principle is safeguarded by the U.S. Constitution, which prohibits such laws because they violate the fairness of legal standards. When a law is enacted that criminalizes behavior retroactively, it undermines the security and predictability necessary for individuals to understand the law as it applies to their actions.

In contrast, the other options describe different legal concepts. The first option refers to laws that might absolve past illegal actions, which is not the essence of ex post facto laws. The second option about altering rules of evidence relates to procedural changes rather than retroactive criminalization. The last option discusses leniency in penalties, which similarly does not capture the core principle of retrospective punishment that defines ex post facto legislation. Thus, the correct identification of an ex post facto law as a statute that criminalizes actions after they were performed reinforces the protective nature of the legal system against arbitrary punishment.