'Ex post facto law' definitions:

Definition of 'Ex post facto law'

From: GCIDE

Ex post facto \Ex" post` fac"to\, or Ex postfacto \Ex" post`fac"to\ ([e^]ks" p[=o]st" f[a^]k"t[-o]). [L., from what is done afterwards.] (Law) From or by an after act, or thing done afterward; in consequence of a subsequent act; retrospective.
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. [1913 Webster]