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Longman Dictionary of Contemporary English
ex post facto law
▪ No bill of attainder, or ex post facto law shall be passed.
The Collaborative International Dictionary
Ex post facto law

Ex post facto \Ex" post` fac"to\, or ||Ex postfacto \Ex" post`fac"to\ ([e^]ks" p[=o]st" f[a^]k"t[-o]). [L., from what is done afterwards.] (Law) From or by an after act, or thing done afterward; in consequence of a subsequent act; retrospective.

Ex post facto law, a law which operates by after enactment. The phrase is popularly applied to any law, civil or criminal, which is enacted with a retrospective effect, and with intention to produce that effect; but in its true application, as employed in American law, it relates only to crimes, and signifies a law which retroacts, by way of criminal punishment, upon that which was not a crime before its passage, or which raises the grade of an offense, or renders an act punishable in a more severe manner that it was when committed. Ex post facto laws are held to be contrary to the fundamental principles of a free government, and the States are prohibited from passing such laws by the Constitution of the United States.

Ex post facto law

An ex post facto law (corrupted from ) is a law that retroactively changes the legal consequences (or status) of actions that were committed, or relationships that existed, before the enactment of the law. In criminal law, it may criminalize actions that were legal when committed; it may aggravate a crime by bringing it into a more severe category than it was in when it was committed; it may change the punishment prescribed for a crime, as by adding new penalties or extending sentences; or it may alter the rules of evidence in order to make conviction for a crime likelier than it would have been when the deed was committed. Conversely, a form of ex post facto law commonly called an amnesty law may decriminalize certain acts. A pardon has a similar effect, in a specific case instead of a class of cases. Other legal changes may alleviate possible punishments (for example by replacing the death sentence with lifelong imprisonment) retroactively. Such legal changes are also known by the Latin term in mitius.

A law may have an ex post facto effect without being technically ex post facto. For example, when a previous law is repealed or otherwise nullified, it is no longer applicable to situations to which it had been, even if such situations arose before the law was voided. The principle of prohibiting the continued application of such laws is called nullum crimen, nulla poena sine praevia lege poenali, especially in European Continental systems. This is related to the principle of legality.

Some common-law jurisdictions do not permit retroactive criminal legislation, though new precedent generally applies to events that occurred before the judicial decision. Ex post facto laws are expressly forbidden by the United States Constitution in Article 1, Section 9, Clause 3 (with respect to federal laws) and Article 1, Section 10 (with respect to state laws). In some nations that follow the Westminster system of government, such as the United Kingdom, ex post facto laws are technically possible, because the doctrine of parliamentary supremacy allows Parliament to pass any law it wishes. In a nation with an entrenched bill of rights or a written constitution, ex post facto legislation may be prohibited.