An ex post facto law (from the
Latin for "from something done afterward") or retrospective law, is a
law that retrospectively changes the legal consequences of acts committed or the legal status of facts and relationships that existed prior to the enactment of the law. In reference to
criminal law, it may
criminalize actions that were legal when committed; or it may aggravate a
crime by bringing it into a more severe category than it was in at the time it was committed; or it may change or increase the punishment prescribed for a crime, such as by adding new penalties or extending terms; or it may alter the
rules of evidence in order to make conviction for a crime more likely than it would have been at the time of the action for which a defendant is prosecuted. Conversely, a form of ex post facto law commonly known as an
amnesty law may decriminalize certain acts or alleviate possible punishments (for example by replacing the death sentence with life-long imprisonment) retrospectively.
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