Definition - Noun : a civil or criminal law with retroactive effect esp : a law that retroactively alters a defendant's rights esp. by criminalizing and imposing punishment for an act that was not criminal or punishable at the time it was committed, by increasing the severity of a crime from its level at the time the crime was committed, by increasing the punishment for a crime from the punishment imposed at the time the crime was committed, or by taking away from the protections (as evidentiary protection) afforded the defendant by the law as it existed when the act was committed Ex post facto laws are prohibited by Article I, Section 9 of the U.S. Constitution.
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