Origin of ex post factoL, from (the thing) done afterward
The definition of ex post facto is something that affects things that happened in the past.
When a law changes the zoning rules and applies even to zoning decisions that were made in the past, it is an example of an ex post facto change to the zoning laws.
ex post facto
Formulated, enacted, or operating retroactively: “When we apply today's morality to yesterday's mores, we indulge in ex post facto judgment” ( William Safire )
Origin of ex post factoLatin ex postfactō ex from postfactō ablative of postfactum that which is done afterward