Definition from Nolo’s Plain-English Law Dictionary
1) The name used for the principal trial court in many states.
2) In the federal system, the term may refer to courts within the 13 circuits. Eleven of these circuits cover different geographic 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, 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.