Glossary
Jurisdiction
A court's authority to hear a particular kind of case involving particular parties. A court without jurisdiction has no power to issue a binding decision: even if both sides want it to.
Jurisdiction has two main components. Subject matter jurisdiction is whether the court is allowed to hear this kind of case (e.g., federal courts can hear federal-law cases but generally not state-law disputes between non-diverse parties). Personal jurisdiction is whether the court has authority over the specific defendant: based on where they live, where they did business, or where the events occurred.
A jurisdictional defect is fatal. If raised at any time: even on appeal, even after a verdict: it can void everything the court did. That's why jurisdiction is one of the first issues lawyers think about when looking at a case.