noun

• (diplomacy) A final statement of terms or conditions made by one party to another, especially one that expresses a threat of reprisal or war if the terms are not met before some specified date and time.
  ". . . But he was directed to present a memorial, urging many reasons for the restitution of the captures made before the war, to represent in its full force, the benefit that would arise, both to France and England, from the total desertion of the war in Germany, and if England should refuse the conditions, now offered as an ultimatum, to wait for farther instructions. The ultimatum arrived in London August 8. M. Buffy soon after wrote to Mr. Pitt, and in his answer, dated August 15, says, that, as to the stile of the ultimatum and letter, the king his master adheres both to the form and substance of them, he laments that peace appears by the proposals and conduct of France to be far distant, and retorts some charges and elusion and delay. M. Buffy, and Mr. Pitt however, had a conference on the two ultimatums jointly, and on the 30th of August an answer to the French ultimatum was delivered, in which the French propositions are agreed to, except in the following particulars. . ."
• (diplomacy, dated) An intended final statement of terms or conditions made by one party to another in a negotiation.
  "1724 William Wynne: The Life of Sir Leoline Jenkins, Judge of the High-Court of Admiralty . . . Thirdly, That they, the Dutch, 5 or 6 Weeks after they had received these Letters, (of 12th of July) call'd for an Ultimatum, so they termed the dernier mot, from us; and that we gave our Ultimatum to the Mediators first by Word of Mouth in August, then on the 5th of September, N. S. in writing."