What is an ex post facto law?

An ex post facto law is defined as a law that makes an act illegal retroactively, meaning that it applies to actions that occurred before the law was enacted. The significance of this type of law lies in the principle of fairness and justice, as it holds individuals accountable for actions that were not deemed illegal at the time they were committed.

In the context of legal systems, ex post facto laws are prohibited by the Constitution in the United States to protect individuals from being punished for actions that were legal when they occurred. This prohibition helps ensure that the law is applied consistently and that individuals are not unfairly penalized for behavior that they could not have known was wrong at the time.

Other choices refer to different legal concepts: making an act illegal in the future does not capture the retroactive nature of ex post facto laws; double jeopardy specifically addresses the issue of being tried twice for the same offense, which is not related to retroactive legality; and expunging previous convictions pertains to the removal of criminal records, which again does not involve the concept of retroactive legal application. Thus, the correct understanding of an ex post facto law is that it is one that applies retroactively, holding individuals accountable for past actions based on newly established laws.