/ɛks pəʊst ˈfæktəʊ/ – noun
Definition: 1. sau sự việc, sau sự kiện;
2. hiệu lực hồi tố, hiệu lực về trước (của luật pháp).
A more thorough explanation: usually used to refer to retroactive laws, that is, laws which change the legal consequences of acts which were committed before the law was made
Example: The new law cannot be applied ex post facto to punish individuals for actions that were legal at the time they were committed.