Personal jurisdiction is the power of a court over the parties in the case. Before a court can exercise power over a party, the constitution requires that the party have certain minimum contacts with the forum in which the court sits. International Shoe v Washington, 326 US 310 (1945). Personal jurisdiction is generally waiveable, so if a party appears in a court without objecting to the court's lack of jurisdiction over it, that objection is forfeited. See Federal Rule of Civil Procedure 12(a)(4).
See also in personam