Latin for "from a thing done afterward." Ex post facto is most typically used to refer to a criminal law that applies retroactively, thereby criminalizing conduct that was legal when originally performed. Two clauses in the US Constitution prohibit ex post facto laws: Art 1, § 9 and Art. 1 § 10. see, e.g. Collins v. Youngblood, 497 US 37 (1990) and California Dep't of Corrections v. Morales, 514 US 499 (1995).