Qui tam is the abbreviation for the Latin phrase “qui tam pro domino rege quam pro se ipso in hac parte sequitur,” meaning "Who sues on behalf of the King as well as for himself."
In a qui tam action, a relator brings an action against a person or company on the government's behalf. The government, not the relator, is considered the plaintiff. If the government succeeds, the relator bringing the suit receives a share of the award. This is also called a popular action.
For example, the federal False Claims Act authorizes qui tam actions against parties who have defrauded the federal government. If successful, a relator in a False Claims Act qui tam action may receive up to 30% of the government's award.
[Last updated in June of 2022 by the Wex Definitions Team]