An ultimatum 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. 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.