What is Ex Post Facto?

Legal Definition
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).

See Constitutional law
Legal Definition
Contracts, crim. law. This is a technical expression, which signifies, that something has been done after another thing, in relation to the latter.

2. An estate granted, may be made good or avoided by matter ex post facto, when an election is given to the party to accept or not to accept. 1 Co . 146.

3. The Constitution of the United States, art. 1, sec. 10, forbids the states to pass any ex post facto law; which has been defined to be one which renders the act punishable in a manner in which it was not punishable when it was committed. 6 Cranch, 138. This definition extends to laws passed after the act, and affecting a person by way of punishment of that act, either in his person or estate. 3 Dall. 386; 1 Blackf. Ind. R. 193 2 Pet. U. S. Rep. 413 1 Kent, Com. 408; Dane's Ab. Index, h. t.

4. This prohibition in the constitution against passing ex post facto law's, applies exclusively to criminal or penal cases, and not to civil cases. Serg. Const. Law, 356. Vide 2 Pick. R. 172; 11 Pick. R. 28; 2 Root, R. 350; 5 Monr. 133; 9 Mass. R. 363; 3 N. H. Rep. 475; 7 John. R. 488; 6 Binn. R. 271; 1 J. J. Marsh, 563; 2 Pet. R. 681; and the article Retrospective.
-- Bouviers Law Dictionary
Legal Definition
After the fact; by an act or fact occurring after some previous act or fact, and relating thereto; by subsequent matter; the opposite of ab initio. Thus, a deed may be good ab initio, or, if invalid at its inception, may be confirmed hy matter ex post facto.
-- Black's Law Dictionary
Legal Definition
"From a thing done afterward". Commonly said as "after the fact."
Legal Definition
After the act (has been committed).
-- Ballentine's Law Dictionary