An ex post facto law is any law passed that makes an activity illegal retroactively. Ex post facto is Latin for “after the fact,” and it typifies the common notion that a person should not be subjected to arbitrary laws, imposed by a government that can decide an action is illegal without notice and as an excuse to exert dominion and control over a person in contravention to his basic human rights. The concept is applied in culturally-specific ways in democratic jurisdictions around the world, but the basic theory behind the principal is consistent.
Retroactive laws are laws that are passed and then applied to activity that occurred in the past. For example, a jurisdiction passes a seatbelt law after an accident occurs where a parent had a child in a car without a seatbelt. The parent cannot be charged with violating the seatbelt law because it did not go into affect until after the accident. Most jurisdictions impute knowledge of all laws on the books to citizens, and can charge a person with a crime even if he had no actual knowledge he was in violation of the law. This establishes a proactive responsibility on the part of the citizen to know the law and to stay within its bounds, and removes from the government the responsibility of trying to figure out what was in the mind of any particular offender.
Imputed knowledge of the law is much more tenable than retroactive application of law in democratic societies. Basic principals of democracy hold that citizens should be free from unreasonable search and seizure by the government. Ex post facto law undermines those basic principals by making government action arbitrary and without basis in a legislative process empowered by the will of the people.
In the U.S., for example, the prohibition against ex post facto law is codified in the U.S. Constitution. The federal government and every state in the union are expressly restricted from passing retroactive laws by what is commonly known as the Ex Post Facto Clause of Article I. This restriction, however, has been limited by the U.S. Supreme Court to apply only to criminal laws and is further defined within that legal area.
The court has established that the ex post facto restriction on laws in the U.S. does not apply to any law or regulation that does not have a punitive intent. Hence, the constitutional clause affects criminal, not civil, law. Within the criminal context, the government can retroactively establish a law that decreases a penalty for an offense, but any retroactive increase in penalty would be punitive and not allowed. The court has held that laws cannot be passed that limit defenses that were available to a defendant at the time a crime was committed.