Latin: after the fact; retroactive.
Legislation is called ex post facto if the law attempts to extend backwards in time and punish acts committed before the date of the law's approval; acts that were otherwise not prohibited at the time committed.
Such laws are constitutionally prohibited in most modern democracies.
For example, the USA Constitution prohibits ex post facto laws.
Two developments have occurred in the United States in regards to the ex post facto constitutional prohibition.
For one, the ex post facto law prohibition does not appear to apply to statutes within the civil law realm; only as regards criminal or penal legislation.
Secondly, the ex post facto principle has been extended not only to prevent the state from punishing an act that was innocent at the time it was committed, but also from increasing retroactively the penalty for a prohibited act. Note that a government is entitled to decrease the severity of a sentence between the time an act is committed in the time of sentencing.
Similarly, it has been used to prevent governments from eliminating defenses that existed and would have been available to the accused at the time the criminal act occurred.
Many cases exist where the government has been taken to task for allegedly increasing the penalty.
For example, litigants have argued that substituting a death penalty sentence of hanging (noose) with a method by electric chair violates the ex post facto principle.