A bushing is a hollow insulating liner through which a conductor may pass. Bushings appear on switchgear

Switchgear

The term switchgear, used in association with the electric power system, or grid, refers to the combination of electrical disconnects, fuses and/or circuit breakers used to isolate electrical equipment. Switchgear is used both to de-energize equipment to allow work to be done and to clear faults...

, transformer

Transformer

A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...

s, circuit breaker

Circuit breaker

A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow...

s and other high voltage equipment.
The bushing is a hollow insulator, allowing a conductor to pass along its centre and connect at both ends to other equipment.
Bushings are often made of wet-process fired porcelain

Porcelain

Porcelain is a ceramic material made by heating raw materials, generally including clay in the form of kaolin, in a kiln to temperatures between and...

, and may be coated with a semi-conducting glaze to assist in equalizing the electrical stress along the length of the bushing.

The inside of the bushing may contain paper insulation and the bushing is often filled with oil to provide additional insulation.