An order that uses the mandatory powers of a court to prohibit a party from issuing or initiating legal proceedings in another court, with the threat of holding the party in contempt and imposing fines and/or other punishments on a party that ignores it. Historically, anti-suit injunctions were most often issued against certain US civil laws that claimed extraterritorial jurisdiction, such as the antitrust laws.
Generally anti-suit injunctions are only issued by courts when they have a case or proceeding before them and one of the parties threatens or otherwise indicates an intent or likelihood of filing competing proceedings on the same issue in another jurisdiction. In modern proceedings anti-suit injunctions are most commonly granted when the second suit or proceeding would be contrary to an exclusive jurisdiction clause or provision in a contract or statute; thus for example if a contract specified arbitration, or that cases be heard by a specific court (e.g., the English High Court, the courts of New York) and one of the parties tries to bring a suit in another court. See, Comity, Lis Pendens Rule.