An ultimatum is a warning to someone that unless they act in a particular way, action will be taken against them.
例句
They issued an ultimatum to the police to rid an area of racist attackers, or they will take the law into their own hands.他們向警方發出了最后通牒，要求肅清整個地區的種族主義暴徒，否則他們將自行采取行動。...a 48-hour ultimatum.期限為48小時的最后通牒