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The phrase ex post facto is Latin for “after the fact”. It generally refers to retroactive acts or laws which change the legal consequences of acts which occurred prior such an act or law. The phrase ex post facto is most typically used in connection with the passage of laws. A law may have an ex post facto effect without being technically ex post facto. Where a law repeals a previous law, the repealed legislation no longer applies to the situations it once did, even if such situations arose before the law was repealed.

In many jurisdictions, ex post facto criminal laws are seen as a violation of the rule of law as it applies in a free and democratic society. Most common law jurisdictions do not permit retroactive criminal legislation under the theory that it is unfair to punish a person for an act which was legal at the time it was committed.

As seen in the example below, however, the use of ex post facto is not limited to the passage of laws.

“The company made an ex post facto ratification of contracts entered into prior to formation.”