What is Ex Post Facto Law?

Legal Definition
An ex post facto law (corrupted from Latin: ex postfacto, lit. 'out of the aftermath') 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 For example, when a previous law is repealed or otherwise nullified, it is no longer applicable to situations to which it had been, even if such situations arose before the law was voided.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.
-- Wikipedia
Legal Definition
A law passed after the occurrence of a fact or commission of an act, which retrospectively changes the legal consequencos or relations of such fact or deed. By Const. U. S. art. 1, § 10, the slates are forbidden to pass "any ex post facto law." In this connection the phrase has a much narrower meaning than its literal translation would justlfy, as will appear from the extracts given below. The phrase "ex post facto," in the constitution, extends to criminal and not to civil cases. And under this head is included:

(1) Every law that makes an action, done before the passing of the law, and which was innocent when done, criminal, and punishes such action.

(2) Every law that aggravates a crime, or makes it greater than it was when committed.

(3) Every law that changes the punishment, and inflicts a greater punishment than the law annexed to the crime when committed.

(4) Every law that niters the legal rules of evidence, and receives less or different testimony than the law required at the time of the commission of the offence, in order to convict the offender. All these, and similar laws, are prohibited by the constitution. But a law may be ex post facto, and still not amenable to this constitutional inhibition ; that is, provided it mollifies, instead of aggravating, the rigor of the criminal law. Boston v. Cummins, 16 Ga. 102, 60 Am. Dec. 717 ; Cummings v. Missouri, 4 Wall. 277, 18 L. Ed. 356; U. S. v. Hail, 2 Wash. C. C. 366, Fed. Cas. No. 15,285; Woart v. Winnick, 3 N. H. 473, 14 Am Else. 384; Calder v. Bull, 3 Dali. 390, 1 L. Ed. 648 ; 3 Story, Const. 2l2. An ex post facto law is one which renders an act punishable, in a manner in which it was not punishable when committed. Such a law may inflict penalties on the person, or pecuniary penalties which swell the public treasury. The legislature is therefore prohibited from passing a law by which a man's estate, or any part of it, shall be seized for a crime, which was not declared, by some previous law, to render him liable to such punishment Fletcher v. Peck, 6 Cranch, 87, 138, 3 In Ed. 162. The plain and obvious meaning of this prohibition is that the legislature shall not pass any law, after a fact done by any citizen, which shall have relation to that fact, so as to punish that which was innocent when done; or to add to the punishment of that which was criminal; or to increase the malignity of a crime; or to retrench the rules of evidence, so as to make conviction more easy. This definition of an ex post facto law is sanctioned by long usage. Strong v. State, 1 Blackf. (Ind.) 196 The term "ex post facto law," in the United States constitution, cannot be construed to include and to prohibit the enacting any law after a fact, nor even to prohibit the depriving a citizen of a vested right to property. Calder v. Bull, 3 Dall. 386, 1 In Ed. 648. "Ex post facto" and "retrospective" are not convertible terms. The latter is a term of wider signification than the former and includes it. All ex post facto laws are necessarily retrospective, but not e converso. A curative or confirmatory statute is retrospective, but not ex post facto. Constitutions of nearly all the slates contain prohibitions against ex post facto laws, but only a few forbid retrospective legislation in specific terms. Black, Const. Prohib. §§ 170, 172, 222. Retrospective laws divesting vested rights are impolitic and unjust; but they are not "ex post facto laws," within the meaning of the constitution of the United States, nor repugnant to any other of its provisions ; and, if not repugnant to the state constitution, a court cannot pronounce them to be void, merely because in their judgment they are contrary to the principles of natural justice. Albee v. May, 2 Pnine, 74, Fed. Cas. No. 134. Every retrospective act is not necessarily an ex post facto law. That phrase embraces only such laws as impose or affect penalties or forfeitures. Locke v. New Orleans, 4 Wall. 172, 18 In Ed. 334. Retrospective laws which do not impair the obligation of contracts, or affect vested rights, or partake of the character of ex post facto laws, are not prohibited by the constitution. Bay v. Gage, 36 Barb. (N. Y.) 447.
-- Black's Law Dictionary
Legal Definition
One which augments the punishment or alters the rules of evidence to the prisoner’s disadvantage, after the commission of the crime. See 45 Am. Rep. 544, note.
-- Ballentine's Law Dictionary