adj. referring to a court's decision or a statute enacted by a legislative body which would result in application to past transactions and legal actions. In criminal law, statutes which would increase penalties or make criminal activities which had been previously legal are prohibited by the constitutional ban on ex post facto laws (Article I, Section 9). Most court decisions which change the elements necessary to prove a crime or the introduction of evidence such as confessions are usually made non-retroactive to prevent a flood of petitions of people convicted under prior rules. Nor can statutes or court decisions take away "vested" property rights or change contract rights. However, some decisions are so fundamental to justice they may have a retroactive effect, depending on the balance between stability of the law and the public good. Retroactive is also called "retrospective."