1) In electronics, a circuit is a path between two or more points along which an electrical current can be carried. (A circuit breaker is a device that interrupts the path when necessary to protect other devices attached to the circuit - for example, in case of a power surge.)
2) In telecommunications, a circuit is a discrete (specific) path between two or more points along which signals can be carried. Unless otherwise qualified, a circuit is a physical path, consisting of one or more wires (or wireless paths) and possibly intermediate switching points. A network is an arrangement of circuits. In a dial-up (switched) connection, a circuit is reserved for use by one user for the duration of the calling session. In a dedicated or leased line arrangement, a circuit is reserved in advance and can only be used by the owner or renter of the circuit.
A virtual circuit, sometimes called a logical circuit, is a path between two or more points that seems like a fixed physical path, but actually is one path out of many possible physical paths that can be arranged. A permanent virtual circuit(PVC) is a virtual circuit that provides a guaranteed connection between two or more points when needed without having to reserve or commit to a specific physical path in advance. This allows many companies to share a common pool of circuits. This approach is used in a frame relay network and offers a committed set of resources to a telephone company customer at a lower price than if the customer leases their own circuits. A switched virtual circuit (SVC) is similar to a permanent virtual circuit, but allows users to dial in to the network of virtual circuits.