The courts
original jurisdiction
The power to hear a case first, as a trial court, rather than on review. The Supreme Court's original jurisdiction is small and set by the Constitution itself.
In full
Article III gives the Supreme Court original jurisdiction over cases affecting ambassadors and those in which a state is a party, most commonly boundary and water disputes between states. Marbury v. Madison (1803) held that Congress cannot enlarge that list by statute, which is the holding that produced judicial review in the first place.
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A plain-language explanation for civic education, not legal advice. When in doubt, the official source named above controls.