Definify.com

a law which operates by after enactment. The phrase is popularly applied to any law, civil or criminal, which is enacted with a retrospective effect, and with intention to produce that effect; but in its true application, as employed in American law, it relates only to crimes, and signifies a law which retroacts, by way of criminal punishment, upon that which was not a crime before its passage, or which raises the grade of an offense, or renders an act punishable in a more severe manner that it was when committed. Ex post facto laws are held to be contrary to the fundamental principles of a free government, and the States are prohibited from passing such laws by the Constitution of the United States.