Ex Post Facto
Definition - What does Ex Post Facto mean?
Ex post facto is a Latin term that means "after the facts." In legal parlance, the term is used in reference to laws that are applied retroactively. In short, actions that were deemed legal before the enactment of the law become illegal or there is an increase to the severity of punishment of an act that was committed prior to enactment of the law.
Justipedia explains Ex Post Facto
In the United States, the Constitution forbids the enactment of ex post facto laws at both the federal and state level. Calder vs. Bull is a case that the U.S. Supreme Court often refers to when deciding ex post facto cases. In this case, Judge Samuel Chase established four types of unconstitutional ex post facto laws and held that ex post facto prohibitions apply only to criminal cases, not civil ones. The Adam Walsh Child Protection and Safety Act of 2006 is one of the laws that, though it can be applied retroactively, is not regarded as unconstitutional.