Retroactive Law and Legal Definition
Retroactive refers to extending the scope or effect to matters that have occurred in the past. In other words it is the application of a given rule to events that took place before the law was in effect. For example, retroactive tax. However most of the countries are guided by the general principle of irretroactivity of law, which forbids this kind of ex-temporary application.
In US, retroactivity applies when a new rule is developed on appeal. In some cases it applies to only current and prospective cases, while in some other cases it applies to cases with complete retroactive effect (including cases in which final judgment has been entered.)