What does "ex post facto" law refer to?

"Ex post facto" laws are specifically defined in the context of legal protection against retroactive prosecution. The term translates from Latin to "after the fact," encapsulating the principle that individuals cannot be punished for actions that were not illegal at the time they were committed. This foundational concept in criminal justice ensures that laws are not applied retroactively to criminalize behavior that was permissible when it occurred. The United States Constitution prohibits the enactment of such laws to protect individuals' rights and maintain fairness in the legal system.

The other choices do not accurately capture the essence of what "ex post facto" laws entail. Increasing penalties for future offenses, mandating new regulations regardless of past laws, or allowing individuals to be tried multiple times for the same offense do not relate to the core principle of punishing actions based on retroactive application of new laws.