An insulator is a material or object which contains no movable electrical charges. When a voltage is placed across an insulator, no charges flow, so no electric current appears.

The term electrical insulator has the same meaning as the term dielectric, but the two terms are often used in different contexts. Conductors and semiconductors, which contain movable charges are the opposite of electrical insulators. Very pure semiconductors are insulators at low temperatures unless doped with impurity atoms that release extra charges which can flow in a current. A few materials (such as silicon dioxide) are almost ideal electrical insulators, a property that is invaluable in flash memory technology. Teflon is another almost ideal insulator, making it a valuable material for long term charge storage in electrets. A much larger class of materials, for example rubber and most plastics are still "good enough" to insulate electrical wiring and cables even though they may have lower bulk resistivity. These materials can serve as practical and safe insulators for low to moderate voltages (hundreds, or even thousands, of volts).