An ultimatum (Latin for 'the last one'; ) is a demand whose fulfillment is requested in a specified period of time and which is backed up by a threat to be followed through in case of noncompliance (open loop). An ultimatum is generally the final demand in a series of requests. As such, the time allotted is usually short, and the request is understood not to be open to further negotiation. The threat which backs up the ultimatum can vary depending on the demand in question and on the other circumstances.

The word is used in diplomacy to signify the final terms submitted by one of the parties in negotiation for settlement of any subject of disagreement. It is accompanied by an intimation as to how refusal will be regarded. English diplomacy has devised the adroit reservation that refusal will be regarded as an "unfriendly act", a phrase which serves as a warning that the consequences of the rupture of negotiations will be considered from the point of view of forcing a settlement. This opens up a variety of possibilities, such as good offices, mediation, the appointment of a commission of inquiry, arbitration, reprisals, pacific blockade and war.