Ex post facto

Etymology

A Latin phrase meaning "from a thing done afterward," from ex, "from, out of," post, "after," and facto, the ablative of factum, "a deed, a thing done." The literal sense is "after the fact." It describes a law reaching backward in time, and constitutional law generally forbids such laws in the criminal sphere.

adverb · /eks-pohst-FAK-toh/
Applied retroactively to acts that happened before the law existed.
What it names
The reason a government cannot pass a law today and then punish you for something that was legal when you did it — a protection written into many constitutions.
Example
The court struck down the penalty as an ex post facto punishment for conduct that had been legal at the time.
Related idea
An ex post facto law changes the rules retroactively; a precedent also reaches back to a past decision, but only to guide present cases consistently, not to punish.