Ex post facto

ex post facto adj. Latin for "after the fact," which refers to laws adopted after an act is committed making it illegal although it was legal when done, or increases the penalty for a crime after it is committed. Such laws are specifically prohibited by the U. S. Constitution, Article I, Section 9. Therefore, if a state legislature or Congress enact new rules of proof or longer sentences, those new rules or sentences do not apply to crimes committed before the new law was adopted.

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EX POST FACTO, 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.