Final answer:
An ex post facto law is a law that has a retroactive effect, making something a crime after it has been committed. It is prohibited by the Constitution to ensure individuals are warned ahead of time that their actions are illegal.
Step-by-step explanation:
An ex post facto law is a law that has a retroactive effect, meaning it can be used to punish crimes that were not considered crimes at the time they were committed or to increase the severity of punishment after the fact. It prohibits making something a crime after it has been committed.
This concept is protected by the Constitution to ensure that individuals are warned ahead of time that their actions are illegal.