Dictionary.net

The name used for the principal trial court in many states. In the federal system, appellate courts are organized into thirteen circuits. Eleven of these cover different geographical areas of the country -- for example, the United States Court of Appeal for the Ninth Circuit covers Alaska, Arizona, California, Hawaii, Idaho, Montana, Nevada, Oregon and Washington. The remaining circuits are the District of Columbia Circuit and the Federal Circuit, (which hears patent, customs and other specialized cases based on subject matter). The term derives from an age before mechanized transit, when judges and lawyers rode "the circuit" of their territory to hold court in various places.