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]