The Collaborative International Dictionary of English v.0.48:

Dielectric \Di`e*lec"tric\, n. [Pref. dia- + electric.] (Elec.)
   Any substance or medium that transmits the electric force by
   a process different from conduction, as in the phenomena of
   induction; a nonconductor, separating a body electrified by
   induction, from the electrifying body.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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

Moby Thesaurus II by Grady Ward, 1.0:

18 Moby Thesaurus words for "dielectric":
   conductance, conduction, conductivity, conductor,
   electric conduction, gas conduction, insulator, ionic conduction,
   liquid conduction, metallic conduction, mho, nonconducting,
   nonconductive, nonconductor, photoconduction, semiconductor,
   superconductivity, superconductor