Definitions:

1. A final demand or statemnt of terms, the rejection of which will result in retaliation or a breakdown in relations. NOUN
   • Diplomats delivered an ultimatum demanding immediate withdrawal of troops.
2. A final statement of terms or conditions made by one party to another, especially one hat expresses a threat of reprisal or war. NOUN
   • He issued an ultimatum, demanding that the project be completed by Friday or face severe consequences.
3. The final propositions, conditions, or terms, offered by either of the parties in a diplomatic negotiation. NOUN
   • The ultimatum left little room for negotiation, putting pressure on both sides to find a resolution quickly.

Synonyms:

• decree
• final demand
• last offer
• warning