Source : Webster's Revised Unabridged Dictionary (1913)

Ultimatum \Ul`ti*ma"tum\, n.; pl. E. {Ultimatums}, L.
   {Ultimata}. [NL. See {Ultimate}.]
   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 a negotiator can offer, the rejection of
   which usually puts an end to the hesitation.

Source : WordNet®

ultimatum
     n : a final peremptory demand
     [also: {ultimata} (pl)]