A law passed after the occurrence of a fact or commission of an act which retrospectively changes the legal consequences or relations of such fact or deed. The Constitution Article 1 Section 10 says that the states are forbidden to pass any ex post facto law. In this connection the phrase has a much narrower meaning than its literal translation would justify, 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 alters the legal rules of evidence, and receives less or different testimony than the law required at the time of the commission of the offense, 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.
Law Dictionary – Alternative Legal Definition
A law passed after the occurrence of a fact or commission of an act, which retrospectively changes the legal consequences or relations of such fact or deed. By Const. U. S. art. 1,