Final answer:
Ex post facto law is discussed in the Constitution of the United States and prohibits criminalizing an act after it has been committed or increasing punishment for a crime after it has been committed.
Step-by-step explanation:
An ex post facto law is discussed in the Constitution of the United States. It is a law that criminalizes an act after it has been committed or increases the severity of punishment for a crime after it has been committed.
The purpose of this prohibition is to ensure that individuals are not punished for actions that were not illegal at the time they were committed.