retroactivity

Retroactive law refers to legislation that applies to acts committed before the law was passed. This can include making past actions criminal, increasing penalties for existing crimes, or altering procedural rules to the disadvantage of the accused.12

The U.S. Constitution prohibits Congress and states from enacting such laws, particularly in criminal matters, to ensure individuals are punished according to established standards. While generally viewed negatively, especially when imposing penalties or taxes, retroactive application can be accepted, such as in the case of a retroactive pay raise. The principle against retroactive criminal laws is a cornerstone of many legal systems, rooted in the idea that punishment should only follow a pre-existing law.23