The term 'ex post facto' is a Latin phrase meaning 'after the fact'. It is used to describe laws or regulations that are applied retroactively, meaning they are applied to events that occurred before the law was enacted. The crossword solution for this term is 'RETRO', which is a word derived from the Latin prefix 'RETRO-', meaning 'BACKWARD' or 'backwards in time'. Therefore, 'RETRO' is an appropriate solution for the term 'ex post facto' as it implies the idea of applying something after the fact.