ultimatum

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A final statement of terms made by one party to another.
2. n. A statement, especially in diplomatic negotiations, that expresses or implies the threat of serious penalties if the terms are not accepted.

Century Dictionary and Cyclopedia

1. n. A final proposal or statement of conditions; especially, in diplomatic negotiations, the final terms of one of the parties, the rejection of which may involve an immediate rupture of diplomatic relations and even lead to a declaration of war.

Wiktionary

1. n. a final statement of terms or conditions made by one party to another, especially one that expresses a threat of reprisal or war

GNU Webster's 1913

1. n. A final proposition, concession, or condition; especially, the final propositions, conditions, or terms, offered by either of the parties in a diplomatic negotiation; the most favorable terms that a negotiator can offer, the rejection of which usually puts an end to the hesitation.
2. n. A final demand, the rejection of which may lead to a resort to force or other compelling action by the party presenting the ultimatum. In international diplomacy, an ultimatum may be used as by the demanding country as a signal to other countries that it gave the other country a seemingly reasonable opportunity to avoid a war; in this way, the demanding country may seek to avoid responsibility for starting a war.

WordNet 3.0

1. n. a final peremptory demand