Source : Webster's Revised Unabridged Dictionary (1913)

   {Ex post facto law}, 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. --Burrill. --Kent.