An ex post facto law (from the Latin for "from something done afterward") or retrospective law, is a law that retrospectively changes the legal consequences of acts committed or the legal status of facts and relationships that existed prior to the enactment of the law. In reference to criminal law, it may criminalize actions that were legal when committed; or it may aggravate a crime by bringing it into a more severe category than it was in at the time it was committed; or it may change or increase the punishment prescribed for a crime, such as by adding new penalties or extending terms; or it may alter the rules of evidence in order to make conviction for a crime more likely than it would have been at the time of the action for which a defendant is prosecuted. Conversely, an ex post facto law, commonly known as an amnesty law, may decriminalize certain acts or alleviate possible punishments (for example by replacing the death sentence with life-long imprisonment) retrospectively.
A law may have an ex post facto effect without being technically ex post facto. For example, when a law repeals a previous law, the repealed legislation no longer applies to the situations it once did, even if such situations arose before the law was repealed. The principle of prohibiting the continued application of 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 occurred prior to the judicial decision. In some nations that follow the Westminster system of government, such as the United Kingdom, ex post facto laws are technically possible as the doctrine of parliamentary supremacy allows parliament to pass any law it wishes. However, in a nation with an entrenched bill of rights or a written constitution, ex post facto legislation may be prohibited.
Ex post facto is the uncomplimentary characterization of law and legislation that applies retrospectively (i.e. "from a thing done afterward").

Ex post facto laws internationally
Australia has no strong constitutional prohibition on ex post facto laws, though narrow retrospective laws may violate constitutional separation of powers principles. Courts do interpret statutes with a strong presumption that they don't apply retrospectively. Retrospective laws designed to combat tax avoidance were passed in the early 1980s by the Fraser government (see Bottom of the harbour tax avoidance).

Australia
Ex post facto criminal laws are constitutionally prohibited by section 11(g) of the Charter of Rights and Freedoms. However, it is possible that the Parliament of Canada could create such a law by using the authority found in section 33 of the Charter (commonly referred to as the notwithstanding clause) as this permits legislation to override certain provisions of the Charter, including section 11. Also, if the punishment for a crime has varied between the time the crime was committed and the time of a conviction, the convicted person is entitled to the lesser punishment.

Canada
Finland has used ex post facto legislature in 1945, after the World War Two on the trial of the war responsibilities in Finland. A law which made the pre-war politics criminal was passed in order to get the leaders nominated by Stalin sentenced. Generally ex post facto jurisprudence is considered violating the Romano-German judicial system, but it isn't prohibited in the Finnish law.

Finland
Any ex post facto criminal law may only be applied if it benefits the accused person (for instance, if weaker sentences are now applicable but weren't previously applicable); otherwise, prohibited by the Declaration of the Rights of Man and of the Citizen.

France
Article 103 of the German basic law requires that an act may only be punished if it has already been punishable by law at the time it was committed (specifically: by written law, Germany following civil law). The Nuremberg Trials and other post-World War II laws that prosecuted former members of the Nazi party are often accused of being ex post facto laws.

Germany
Article 28I of the Indonesian constitution prohibits trying citizens under retrospective laws in any circumstance. This was tested in 2004 when the conviction of one of the Bali bombers under retrospective anti-terrorist legislation was quashed.

Indonesia
Ex post facto laws, in all contexts, are prohibited by Article 169 (Chapter 11) of the Constitution of Iran.

Italy
The imposition of retrospective criminal sanctions is prohibited by Article 15.5.1° of the Constitution of Ireland. Retrospective changes of the civil law have also been found to violate the constitution when they would have resulted in the loss in a right to damages before the courts, the Irish Supreme Court having found that such a right is a constitutionally protected property right.

Ireland
Article 39 of the Constitution of Japan prohibits the retrospective application of laws. Article 6 of Criminal Code of Japan further state that if a new law comes into force after the deed was committed, the lighter punishment must be given.

Japan
Article 97 of the Norwegian constitution prohibits any law to be given retrospective effect.

Norway
Prohibited in criminal law by clause 35.(3)(l) of the Constitution of the Republic of South Africa; an exception exists for offenses which were illegal under international law at the time of commission.

South Africa
Retrospective penal sanctions and other retrospective legal effects of criminal acts due the State are prohibited by chapter 2, article 22, point 5 of the Instrument of Government (Regeringsformen). Retrospective taxes or charges aren't prohibited but can only have retrospective effect reaching back to when a new tax bill was proposed by the government, the retrospective effect thus reaches from that time until the bill is passed by the parliament.

Sweden
Ex post facto punishment is prohibited by Article 38 of the Constitution of Turkey.

Turkey
Ex post facto laws are strictly frowned upon, but are permitted by virtue of the doctrine of parliamentary sovereignty. Ex post facto criminal laws are prohibited by Article 7 of the European Convention on Human Rights, to which the United Kingdom is a signatory, but parliamentary sovreignty takes priority even over this.

United Kingdom
Prohibited by Article I section 9 (applying to federal law) and section 10 (applying to state law) of the U.S. Constitution. Over the years, when deciding ex post facto cases, the United States Supreme Court has referred repeatedly to its ruling in the Calder v. Bull case of 1798, in which Justice Chase established four categories of unconstitutional ex post facto laws. A current U.S. law that definitely has an ex post facto effect is the Adam Walsh Child Protection and Safety Act of 2006. This law, which imposes new registration requirements on convicted sex offenders, gives the U.S. Attorney General the authority to apply the law retrospectively.
Another example is the firearms prohibitions added on misdemeanor domestic violence offenders and subjects of restraining orders - the punishment for their crimes and acts (restraining orders require no criminal conviction) now includes being sentenced to up to 10 years if one possesses a firearm - even those that were legally possessed them at the time the law was passed. Many police lost their jobs as a result. This is legally upheld because it is considered regulatory, not punitive - it is a status offense.
See also: Fourteenth Amendment to the United States Constitution, Fourteenth Amendment, Bouie v. City of Columbia, and Rogers v. Tennessee

United States
Most European nations, and all European Union nations, are bound by the European Convention on Human Rights. Article 7 of the convention prohibits ex post facto criminal laws subject to two exceptions. It also prohibits a heavier penalty being imposed than was applicable at the time when an act was committed. The exceptions are:

Acts illegal under international law at the time of their commission.
Acts criminal according to "the general principles of law recognized by civilized nations". Quotations
To students of Latin, this phrase doesn't appear to make sense, as it consists of the preposition ex, the preposition post, and a noun with the wrong grammatical case to agree with post. Indeed, the Latin for this phrase is actually two words, ex postfacto, literally, out of a postfactum (an after-deed), or more naturally, from a law passed afterward.
Therefore, ex post facto or ex postfacto is natively an adverbial phrase, a usage demonstrated by the sentence He was convicted ex post facto (i.e., from a law passed after his crime). The law itself would rightfully be a postfactum law (lex postfacta); nevertheless, despite its redundant or circular nature, the phrase an ex post facto law is used.