A more thorough explanation:

Definition: Post facto (pohst fak-toh) means "after the fact." It is often used in legal contexts to refer to actions taken after an event has already occurred.

Ex Post Facto: Ex post facto (eks pohst fak-toh) is a related term that means "from a thing done afterward." It refers to actions taken retroactively, or after the fact.

Example: A law that makes an action illegal after it has already been committed is an ex post facto law. For example, if a person is arrested for a crime that was not illegal at the time it was committed, but was made illegal by a new law passed after the fact, that law would be considered ex post facto.

Explanation: The example illustrates how ex post facto laws can be unfair because they punish people for actions that were not illegal at the time they were committed. This violates the principle of "no punishment without a law," which is a fundamental principle of justice.