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.