Generally refer to laws or court decisions which affect already existing rights, obligations and duties, or attach new conditions in respect to transactions or events already past.
Retroactivity depends on whether the new provision attaches new legal consequences to events completed before its enactment. Landgraf v. USI Film Prods., 114 S. Ct. 1483, 1499 & n.3 (1994). "The conclusion that a particular rule operates `retroactively' comes at the end of a process of judgment concerning the nature and extent of the change in the law and the degree of connection between the operation of the new rule and a relevant past event." Id. at 1499.Harper v. Virginia Dep't of Taxation, 113 S. Ct. 2510, 2517 (1993) holds that no court may refuse to apply rule of federal law retroactively once the Court applies it to the parties before it.
A statute passed by a legislature usually states that it shall only apply after a certain date. Occasionally, though, laws are made retroactive--that is, they apply to events that happened before the law was passed. (Criminal laws are never retroactive--the legislature cannot make a past act a crime.) If the statute itself doesn't indicate the date it is to become effective, courts normally interpret it to have future effect only.Example: Assume that the Minnesota legislature passes a law requiring a couple to undergo a blood test as a requirement of being married. The legislature intends that it apply to couples both planning to marry and already married. (Currently, Minnesota does not require blood tests before marriage.) Applying the law to already married couples is a retroactive application of the law. A married Minnesotan might challenge the law arguing that applying it to her is unconstitutional because she had no expectation of ever having to undergo a blood test as a requirement of being married in Minnesota.