What is Personal Jurisdiction?

Legal Definition
Personal jurisdiction is a court's jurisdiction over the parties to a lawsuit, as opposed to subject-matter jurisdiction, which is jurisdiction over the law and facts involved in the suit. If a court does not have personal jurisdiction over a party, its rulings or decrees cannot be enforced upon that party, except by comity; i.e., to the extent that the sovereign having jurisdiction over the party allows the court to enforce them upon that party. A court that has personal jurisdiction has both the authority to rule on the law and facts of a suit and the power to enforce its decision upon a party to the suit. In some cases, territorial jurisdiction may also constrain a court's reach, such as preventing hearing of a case concerning events occurring on foreign territory between two citizens of the home jurisdiction.
-- Wikipedia
Legal Definition
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