The Collaborative International Dictionary of English v.0.48:

Nonconductor \Non`con*duct"or\, n. (Physics)
   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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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