The Collaborative International Dictionary of English v.0.48:

Ultimatum \Ul`ti*ma"tum\ ([u^]l`t[i^]*m[=a]"t[u^]m), n.; pl. E.
   Ultimatums ([u^]l`t[i^]*m[=a]"t[u^]mz), L. Ultimata. [NL.
   See Ultimate.]
   1. 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.
      [1913 Webster]

   2. 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.
      [PJC]
      [1913 Webster]

WordNet (r) 3.0 (2006):

ultimatum
    n 1: a final peremptory demand

Moby Thesaurus II by Grady Ward, 1.0:

84 Moby Thesaurus words for "ultimatum":
   admonishment, admonition, alarm, blackmail, boundary condition,
   call, call for, catch, caution, caveat, claim, clause, condition,
   conditions, contribution, demand, demand for, demands,
   deterrent example, donnee, draft, drain, duty, escalator clause,
   escape clause, escape hatch, exaction, example, extortion,
   extortionate demand, final notice, final offer, final warning,
   fine print, firm price, given, grounds, heavy demand, hint,
   imposition, impost, indent, insistent demand, joker, kicker,
   last word, lesson, levy, limiting condition, monition, moral,
   nonnegotiable demand, notice, notification, object lesson,
   obligation, order, parameter, prerequisite, provision, provisions,
   proviso, requirement, requisite, requisition, rush, rush order,
   saving clause, sine qua non, small print, specification,
   stipulation, stipulations, string, tax, taxing, terms, threat,
   tip-off, tribute, verbum sapienti, warning, warning piece,
   whereas



Bouvier's Law Dictionary, Revised 6th Ed (1856):

ULTIMATUM. The last proposition made in making a contract, a treaty, and the
like; as, the government of the United States has given its ultimatum, has
made the last proposition it will make to complete the proposed treaty. The
word also means the result of a negotiation, and it comprises the final
determination of the parties concerned in the object in dispute.



The Devil's Dictionary (1881-1906):

ULTIMATUM, n.  In diplomacy, a last demand before resorting to
concessions.
    Having received an ultimatum from Austria, the Turkish Ministry
met to consider it.
    "O servant of the Prophet," said the Sheik of the Imperial Chibouk
to the Mamoosh of the Invincible Army, "how many unconquerable
soldiers have we in arms?"
    "Upholder of the Faith," that dignitary replied after examining
his memoranda, "they are in numbers as the leaves of the forest!"
    "And how many impenetrable battleships strike terror to the hearts
of all Christian swine?" he asked the Imaum of the Ever Victorious
Navy.
    "Uncle of the Full Moon," was the reply, "deign to know that they
are as the waves of the ocean, the sands of the desert and the stars
of Heaven!"
    For eight hours the broad brow of the Sheik of the Imperial
Chibouk was corrugated with evidences of deep thought:  he was
calculating the chances of war.  Then, "Sons of angels," he said, "the
die is cast!  I shall suggest to the Ulema of the Imperial Ear that he
advise inaction.  In the name of Allah, the council is adjourned."