The term most typically used to refer to a law that applies retroactively, thereby criminalizing conduct that was legal when originally performed. But I'd think that it also would apply to a law that lets the president off the hook by retroactively declaring something (such as warrant-less wiretapping or torture) to be legal.
Also, one might argue that retroactively allowing torture would, in effect, change the legal status of "enemy combatants" after the fact.
Are there any Constitutional scholars out there who could weigh in?Here are a couple definitions I found:
An ex post facto law is a law passed after the occurrence of an event or action which retrospectively changes the legal consequences of the event or action.
http://www.lectlaw.com/def/e086.htm-------------------------------
Ex post facto laws retroactively change the rules of evidence in a criminal case, retroactively alter the definition of a crime, retroactively increase the punishment for a criminal act, or punish conduct that was legal when committed. They are prohibited by Article I, Section 10, Clause 1, of the U.S. Constitution. An ex post facto law is considered a hallmark of tyranny because it deprives people of a sense of what behavior will or will not be punished and allows for random punishment at the whim of those in power.
http://www.answers.com/topic/ex-post-facto-law--------------