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.他们向警方发出了最后通牒，要求肃清整个地区的种族主义暴徒，否则他们将自行采取行动。...a 48-hour ultimatum.期限为48小时的最后通牒