Other forms: insulators
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 word insulator comes from the Latin root insulātus, which means "like an island." If your house is built out of good insulators, you might enjoy an island of warmth inside it even during cold winter nights. The opposite of insulator is conductor: a material that easily transmits heat or electricity.