An ex post facto law (Latin for "from a thing done afterward"), also known as a retrospective law, is a law that is retroactive, i.e. that affects facts or legal relationships that have existed prior to the enactment of the law. Ex post facto is a term from the Latin and means "after the deed." In reference to criminal law, it is an attempt to criminalize an action that was not a crime at the time it was committed. A law may have an ex post facto effect without being technically ex post facto. For example, when a law that repeals another law, the repealed legislation will no longer apply to situations even when those situations arose before the law was repealed. The principle of prohibiting these kinds of laws is also known as Nullum crimen, nulla poena sine praevia lege poenali.
Generally speaking, ex post facto 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 retrospective legislation, though some have suggested that judge-made 'law' is retrospective as a new precedent applies to events that happened prior to the judicial decision. In some nations that follow the Westminister system of government, such as the United Kingdom, ex post facto laws are technically possible as parliamentary supremacy allows the parliament to pass any law it wishes. However, in a nation with an entrenched bill of rights or written constitution this may prohibit ex post facto legislation.