Referring to laws that retroactively criminalize conduct that was legal when performed, increase punishment for past acts, or alter legal rules to the disadvantage of defendants. The Constitution prohibits ex post facto laws to ensure fair notice and prevent legislative persecution.
From Latin 'ex post facto' meaning 'from after the fact.' This principle emerged from English common law's rejection of retroactive criminal legislation, reflecting the fundamental legal maxim that people should know what conduct is illegal before they can be punished for it.
Try Another Word