OLA’S EESE

Depends on the context.

In distribution systems (i.e. the conductors and related equipment between the last substation and houses/buildings), it refers to a circuit that comes out of the substation. Then it may branch into laterals.

In house wiring, where the National Electrical Code governs (in the US), it refers to a circuit between the service equipment (i.e. first main circuit breaker) and any other OCPD (“overcurrent protection device”, i.e. circuit breaker or fuse, usually a circuit breaker). While a service refers to the circuit between the last utility transformer and the service equipment, and a branch circuit refers to a circuit between the last circuit breaker and an outlet (be it a receptacle, a lighting or a switch outlet).