An insulator is a material or an object that does not let electricity, heat, or sound pass through it easily. The plastic around a wire is an insulator: that is why you can touch the outside and not get a shock. Rubber, glass, dry wood, and many plastics are common electrical insulators. Wool, foam, and still air are insulators against heat; thick curtains or foam panels can work as insulators against sound. In school science the pair you learn is conductor (lets electricity through) and insulator (does not). Though actually, nothing is a perfect stop. An insulator just resists the flow very strongly. People also use insulator for the object itself — the ceramic cup on a power line, for example — not only for the kind of material.