The courts
circuit
One of the thirteen regions of the federal appellate system. Saying a case is in the Ninth Circuit names both the court and the states it covers.
In full
Eleven numbered circuits cover groups of states, the D.C. Circuit covers the capital and most challenges to federal agency action, and the Federal Circuit takes certain subject matter nationwide. A circuit's published decisions bind every district court inside it and nobody outside it, which is what makes a circuit split the standard reason for Supreme Court review.
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A plain-language explanation for civic education, not legal advice. When in doubt, the official source named above controls.