What is the thin insulator placed between the plates of a capacitor called?

The thin insulator placed between the plates of a capacitor is known as a dielectric. A dielectric material serves several important functions in a capacitor. Firstly, it increases the capacitor's ability to store electrical energy by allowing it to hold more charge at a given voltage. This happens because the dielectric material polarizes in an electric field, which reduces the effective electric field across the capacitor plates. As a result, the capacitance, which measures a capacitor's ability to store charge, is enhanced.

The presence of a dielectric also helps prevent the plates of the capacitor from directly touching each other, which could lead to short-circuiting. Different dielectric materials have distinct properties that determine the capacitor's overall performance, like voltage rating and frequency response. Thus, the proper selection of dielectric is critical in capacitor design for specific applications.

Other terms mentioned in the options have different meanings; for instance, an insulator is a general term for materials that resist electric current, while a resistor specifically refers to a component that limits current flow. Capacitance is a measure of a capacitor's ability to store charge, but it is not the name for the material itself between the plates. Hence, the term that correctly identifies the insulating material used in capacitors is dielectric.