Retroactive

Having reference to things that happened in the past, prior to the occurrence of the act in question.

A retroactive or retrospective law is one that takes away or impairs vested rights acquired under existing laws, creates new obligations, imposes new duties, or attaches a new and different legal effect to transactions or considerations already past. Common-law principles do not favor the retroactive effect of laws in the majority of cases, and canons of legislative construction presume that legislation is not intended as retroactive unless its language expressly makes it retroactive.

Retroactive criminal laws that increase punishment for acts committed prior to their enactments are deemed EX POST FACTO LAWS and are unenforceable because they violate Article I, Section 9, Clause 3, and Section 10, Clause 1, of the U.S. Constitution and comparable provisions of state constitutions.