From Longman Dictionary of Contemporary Englishex post facto lawex post fac‧to law /ˌeks pəʊst ˈfæktəʊ ˌlɔː $ -poʊst ˈfæktoʊ ˌlɒː/ noun [countable] law SCLa law that makes a particular action into a crime, and then punishes people who took that action before it had legally become a crime
Examples from the Corpus
ex post facto law• No bill of attainder, or ex post facto law shall be passed.Origin ex post facto law (1700-1800) Latin “from a thing done afterward” 