A relator is a private individual who brings a lawsuit on behalf of the government. This term is often used in the context of qui tam actions under the False Claims Act. If the prosecution succeeds, the private party receives a portion of the reward. See False Claims Act.
For example, if an employee of a defense contractor discovered that their boss was defrauding the government, the employee could file a False Claims Act qui tam suit against their employer. In that action, the employee would be the relator, the employer would be the defendant, and the government would be the plaintiff. See, e.g., United States ex rel Eisenstein v. City of New York (08-660).
[Last updated in June of 2024 by the Wex Definitions Team]