Ex Post Facto Law - A law that makes criminal an act that was legal when it was committed, or that increases the penalty for a crime after it has been committed, or that changes the rules of evidence to make conviction easier; a retroactive criminal law. A Latin term meaning "after the fact."
The state legislatures and Congress are forbidden to pass such laws by Article I, section 9 and 10 or the Constitution.
source:
http://www.vote-smart.org/resource_g...hp#expostfacto