In the American legal system "queen for a day" refers to written documents that are designed to create a potential mutually beneficial arrangement with the federal government and a person of interest regarding a criminal investigation. The concept of a "queen for a day", more commonly known as proffer agreements permit the accused individual to disclose to authorities key points of knowledge to crimes committed by that person and/or others, with implied assurance that said knowledge will not be used against them in later proceedings.

More precisely it facilitates a symbiotic relationship by giving prosecutors a sample of the individual's knowledge that in turn provides the accused leverage to bargain with for something such as a lesser sentence or immunity in regards to the whole of the investigation.