ex post facto law

Main Entry: ex post facto law
Function: noun
: a civil or criminal law with retroactive effect; especially : 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
NOTE: Ex post facto laws are prohibited by Article I, Section 9 of the U.S. Constitution.

ex post facto law

law that retroactively makes criminal conduct that was not criminal when performed, increases the punishment for crimes already committed, or changes the rules of procedure in force at the time an alleged crime was committed in a way substantially disadvantageous to the accused.

Learn more about ex post facto law with a free trial on Britannica.com.