noun.
A material that conducts little or no electricity, heat, or sound.
The Century Dictionary and Cyclopedia
noun.
A substance which does not conduct or transmit a particular form of energy (specifically, heat or electricity), or which transmits it with difficulty: thus, wool is a non-conductor of heat; glass and dry wood are non-conductors of electricity. See conductor, 6, electricity, and heat.
the GNU version of the Collaborative International Dictionary of English
noun.
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