You're looking for "ex post facto law": http://en.wikipedia.org/wiki/Ex_post_facto_law http://en.wikipedia.org/wiki/Ex_post_facto_law "In the United States, th

You're looking for "ex post facto law": http://en.wikipedia.org/wiki/Ex_post_facto_law http://en.wikipedia.org/wiki/Ex_post_facto_law "In the United States, the federal government is prohibited from passing ex post facto laws by clause 3 of Article I, section 9 of the U.S. Constitution and the states are prohibited from the same by clause 1 of Article I, section 10. This is one of the very few restrictions that the United States Constitution made to both the power of the federal and state governments prior to the Fourteenth Amendment. " "In the United Kingdom, ex post facto laws are frowned upon, but are permitted by virtue of the doctrine of parliamentary sovereignty."