Criminal laws that are retroactively applied to those who engaged in activities when they were not yet illegal are called

Criminal laws that are retroactively applied to those who engaged in activities when they were not yet illegal are called ex post facto laws. Ex post facto lawsare forbidden to be passed by the Constitution of the United States.
This law may increases the punishment for crimes already committed, or changes the rules of procedure in force at the time an alleged crime was committed in a way substantially disadvantageous to the accused.