In electrical and mechanical engineering, bushing can describe any of a variety of parts which allow one piece to pass through another. Plain bearings are also referred to as bushings.
In mechanical engineering, bushings cushion the truck of an axle. In a skateboard, the bushing can be stiffer or softer. A stiffer bushing provides a more stable ride and prevents wobbling at higher speeds where a softer bushing allows for a softer ride with the ability to carve turns more easily. See also Bush (mechanical).
Bushings are also used to transfer loads from a fastening to a much larger area in the underlying structure, the object being to reduce the strain on individual fibers within the underlying structure. The following site gives some more information about what could be called in-situ bushings. (See the paragraph called 'Axle and hub'.
In electrical engineering a bushing is a component that allows a high voltage conductor to pass safely through an earthed metal wall or casing. They appear on switchgear, transformers, breakers and most other high voltage equipment. Some of the higher voltage types are called capacitor bushings because they form a low value capacitor between the conductor and the wall. This is done in order to reduce the electrical stress that may otherwise occur and cause breakdown failure. Bushings do sometimes fail due to partial discharge degradation in the insulation. There is at present great interest in the electricity supply industries in monitoring the condition of high voltage bushings.