ultimatum

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A final offer or demand made by one party to another, especially in diplomatic negotiations, expressing or implying the threat of serious consequences or the breakoff of relations if the terms are not accepted.

from The Century Dictionary.

• noun 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.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.
• noun 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a final peremptory demand