insulator

from WordNet (r) 3.0 (2006)

insulator
    n 1: a material such as glass or porcelain with negligible
         electrical or thermal conductivity [syn: {insulator},
         {dielectric}, {nonconductor}] [ant: {conductor}]

from The Collaborative International Dictionary of English v.0.48

Insulator \In"su*la`tor\, n.
   1. One who, or that which, insulates.
      [1913 Webster]

   2. (Elec. & Thermotics) A substance or object that insulates;
      a nonconductor; as, polyurethane foam is a popular thermal
      insulator.
      [1913 Webster +PJC]