A lining installed in a fitting to provide protection or insulation.
In an underground tank system, if for example, it is desirable to prevent electrical continuity between a cathodically protected steel tank and a steel pipe connected to the tank. (If there were electrical continuity, the sacrificial anodes attached to the tank, to protect it from corrosion, would have to be capable of protecting the pipe as well as the tank.)
To insulate the tank from the pipe, the fitting which connects the two is lined with a “dielectric bushing,” a plastic fitting that will not conduct electrical current.
See also Double tapped bushing.