noun (n)

• a material such as glass or porcelain with negligible electrical or thermal conductivity(noun.substance)

  Synonym:
  dielectric, insulator
  Antonym:
  conductor
  

  source: wordnet30
• A substance which does not conduct, that is, convey or transmit, heat, electricity, sound, vibration, or the like, or which transmits them with difficulty; an insulator; as, wool is a nonconductor of heat; glass and dry wood are nonconductors of electricity.(noun)

  source: webster1913