Formal/Serious:
* Demand: This implies a strong request that must be met.
* Requirement: This emphasizes a necessary condition.
* Non-negotiable: This highlights that there is no room for compromise.
* Condition: This implies a specific stipulation that must be fulfilled.
* Injunction: This emphasizes a formal order or command.
Less Formal/More Conversational:
* Deadline: This emphasizes a time limit.
* Bottom line: This indicates a final and unchangeable decision.
* Take it or leave it: This is a more colloquial way of saying "ultimatum."
* Dealbreaker: This highlights a condition that could end negotiations.
* Last chance: This emphasizes that this is the final opportunity.
Other options:
* Dichotomy: This suggests a clear choice between two options.
* Impasse: This describes a situation where there is no way forward unless someone gives in.
* Confrontation: This emphasizes a tense situation where a decision must be made.
The best word to use will depend on the specific context and the tone you want to convey.