A law that retroactively makes criminal an act that was not criminal at the time it was done. In the United States, the passage of such laws is forbidden by the Constitution.
The definition of an ex post facto law is a law that applies to crimes that happened before the law was passed.
An example of an ex post facto law is a law passed in 1994 that applies to acts that occurred in 1989.
A law intended to apply to crimes or events that took place before its passage. The United States Constitution forbids the passage of ex post facto criminal laws, on the principle that it is wrong to punish an act which was not illegal when committed.