An insulator is a material that does not allow electric current to flow freely). The atoms of an insulator have tightly bound electrons that cannot move easily. Insulators are used in electrical equipment to support and separate electrical conductors without allowing current through themselves. An insulating material used in bulk to wrap electrical cables or other equipment is called insulation). The term insulator is also used more specifically to refer to insulating supports used to attach electric power distribution or transmission lines to utility poles and transmission towers).

Some examples of insulating materials include paper, plastic, rubber, glass, and air. Vacuum is also an insulator, but it is not actually a material. Most electrical conductors are covered by insulation. Magnet wire, for example, is coated with an extremely thin layer of insulation so that more turns or larger wire may be used in the winding of transformers.

It is important to note that a perfect insulator does not exist because even insulators contain small numbers of mobile charges (charge carriers) which can carry current. In addition, all insulators become electrically conductive when a sufficiently large voltage is applied that the electric field tears electrons away from the atoms. This is known as electrical breakdown, and the voltage at which it occurs is called the breakdown voltage of an insulator).