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.
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.
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.