A circuit is a circular path. It could be the circuit of an electric current or the route of the Indianapolis Motor Speedway. If you’re promoting your new book, you’ll be making the talk show circuit.
What all these circuits have in common the idea of tracing a circle. A circuit preacher doesn't stay at one church, but instead travels from church to church, following a particular route. Originally US judges traveled from town to town, too, and today’s division of the country into twelve judicial circuits is a holdover from that era — "the Third Circuit," for instance, includes Delaware, New Jersey, and Pennsylvania.