Source : Webster's Revised Unabridged Dictionary (1913)

Insulator \In"su*la`tor\, n.
   1. One who, or that which, insulates.

   2. (Elec. & Thermotics) The substance or body that insulates;
      a nonconductor.

Source : WordNet®

insulator
     n : a material such as glass or porcelain with negligible
         electrical or thermal conductivity [syn: {dielectric}, {nonconductor}]
         [ant: {conductor}]