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).
It's an interesting thing, Ex Post Facto laws are prohibited to both the federal government and the States.
It could [and likely will] be argued that the passing of new laws (like illegalizing guns) WITHOUT a retroactive element is not Ex Post Facto; indeed, I would agree. They (gun-banners) needn't worry about Ex Post Pacto prohibitions in the Constitution if they do not get greedy and try to make preceding gun-ownership illegal.
But then again, the 2nd is quite explicit on not infringing the right, and yet we have the GCA and NFA both.