noun
A
statute that treats with facts or occurrences something that took place before
the statute was enacted. While unusual, a retroactive law is only
unconstitutional if it impairs vested rights, interferes with obligations under
contracts (such as creating new obligations or attaching new disabilities), has
the effect of an ex post facto
law or bill of attainder, or is
prohibited by the United States Constitution. Certain decisions granting new
rights to criminal defendants under constitutional law have been given full
retroactive effect. While others have been held to be effective only from the
time of enactment forward. See also ex
post facto and bill of
attainder.