The word ultimatum comes from the Latin, meaning 'final'. In practice, an ultimatum is a final offer containing 'the ultimate' threat, whereby the person being given the ultimatum is required to do something, or take the consequences. Ultimatums, by definition, mean that the negotiation stage is over, and that actions must follow. A diplomatic ultimatum is usually a threat of war, or economic actions against another nation. Historically, ultimatums have usually led to wars, directly or indirectly.
Examples of Ultimatum:
British ultimatum to Germany prior to declaration of war, September 3 1939|
Plea bargain, where the accused is required to accept terms or face a trial.
Georg philipp telemann's ultimatum addressed to the hamburg city council, 3 September 1722.