insulator

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. A material that insulates, especially a nonconductor of sound, heat, or electricity.
2. n. A device that insulates.

Century Dictionary and Cyclopedia

1. n. One who or that which insulates; specifically, a substance or body that interrupts the communication of electricity or heat to surrounding objects; a non-conductor; anything through which an electric current will not pass. The figures show the usual forms of insulators employed in telegraph-lines to support the wire on the post. They are frequently made of porcelain or glass, and in the shape of an inverted cup, round which the wire is wrapped or is attached by a hook depending from it, or the like. In the case of electricity the commonest insulators for supports are glass, porcelain, and vulcanized rubber; and for covering wires conveying currents, silk, cotton, gutta-percha, and rubber. These substances do not absolutely prevent the communication of electricity, but a good glass Leyden jar, for example, will hold a charge for months. No perfect insulator for either electricity or heat is known, and the distinction between conductors and insulators is some what arbitrary.

Wiktionary

1. n. A substance that does not transmit heat (thermal insulator), sound (acoustic insulator) or electricity (electrical insulator).
2. n. A non-conductive structure, coating or device that does not transmit sound, heat or electricity (see image)

GNU Webster's 1913

1. n. One who, or that which, insulates.
2. n. (Elec. & Thermotics) A substance or object that insulates; a nonconductor.

WordNet 3.0

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