Any material that keeps energy such as electricity, heat, or cold from easily transferring through is an insulator. Wood, plastic, rubber, and glass are good insulators. Swiss cheese...not so much.

The idea of an insulator comes from the Latin idea of insulātus, which means "to make into an island." This gives rise to the idea of separation, which works for the idea of an insulator, which effectively blocks energy. If your house is built out of good insulators, you might enjoy an island of warmth inside it even in the dead of winter.

Definitions of insulator

n a material such as glass or porcelain with negligible electrical or thermal conductivity

dielectric, nonconductor
a substance that readily conducts e.g. electricity and heat
an insulating liner in an opening through which conductors pass
mineral wool, rock wool
a light fibrous material used as an insulator
glass wool
glass fibers spun and massed into bundles resembling wool
Type of:
material, stuff
the tangible substance that goes into the makeup of a physical object

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