A material used to inhibit or prevent the conduction of heat or of electricity. Common heat insulators are asbestos, cellulose fibers, feathers, fiberglass, fur, stone, wood and wool; all are poor conductors of heat.

In the conduction of electricity from point to point, the conductor acts as a guide for the electric current and must be insulated at every point of contact with its support to prevent escape, or leakage, of the current.

Good electrical insulators, or dielectrics, include dry air, dry cotton, glass, paraffin, porcelain, resin, rubber and varnishes.