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. (Alternatively, rather than redefining the relevant acts as non-criminal, it may simply prohibit prosecution; or it may enact that there is to be no punishment, but leave the underlying conviction technically unaltered.) A pardon has a similar effect, in a specific case instead of a class of cases (though a pardon more often leaves the conviction itself – the finding of guilt – unaltered, and occasionally pardons are refused for this reason). 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. https://en.m.wikipedia.org/wiki/Ex_post_facto_law