ex post facto
adj. Latin for "after the fact," which refers to laws adopted after an act is committed making it illegal although it was legal when done, or increasing the penalty for a crime after it is committed. Such laws are specifically prohibited by the U.S. Constitution, Article I, Section 9. Therefore, if a state legislature or Congress enacts new rules of proof or longer sentences, those new rules or sentences do not apply to crimes committed before the new law was adopted.