Simple said the electric insulator definition:
An electric insulator is a material which does not conduct electric charges. No electric current can flow through the material.

In the other word, electric insulator definition is
If it was an electrical insulator, the charges would remain fixed wherever they built up, meaning the charge inside the insulator would depend on the charge distribution, as would the strength of the electric field, which would be more than zero.

Elaborate the electric insulator definition
An electric insulator, also called a dielectric, is a material that resists the flow of electric current. An insulating material has atoms with tightly bonded valence electrons. These materials are used in parts of electrical equipment, also called insulators or insulation, intended to support or separate electrical conductors without passing current through themselves, such as porcelain pin type insulator. The term is also used more specifically to refer to insulating supports that attach electric power transmission wires to utility poles or pylons.

If you want to learn more about all type insulators definition, for example what the porcelain pin insulator is.

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