Concept:
A capacitor consists of two conducting plates separated by an insulating medium called dielectric.
Step 1: The two plates of a capacitor are conductors.
Step 2: The material between the plates should not allow direct conduction of current.
Step 3: Therefore, an insulating material is placed between the plates.
Step 4: This insulating material is called dielectric.
Hence:
\[
\boxed{\text{Insulating material}}
\]