Nolo's Plain-English Law Dictionary

Ex Post Facto

Latin for "after the fact." Refers to laws adopted after an act is committed, making it illegal retroactively. Or, it can refer to laws that increase the penalty for a crime after it is committed. Such laws are specifically prohibited by the U.S. Constitution, Article I, Section 9.
NOLO-web2:DRU1.6.12.2.20161011.41205